scripts: dbgbmap[d3].py: Disabled gcksum checking by default

By default, we don't actually do anything if we find an invalid gcksum,
so there's no reason to calculate it everytime.

Though this performance improvement may not be very noticeable:

  dbgbmap.py w/  crc32c lib w/  no_ck --no-ckdata: 0m0.221s
  dbgbmap.py w/  crc32c lib w/o no_ck --no-ckdata: 0m0.269s
  dbgbmap.py w/o crc32c lib w/  no_ck --no-ckdata: 0m0.388s
  dbgbmap.py w/o crc32c lib w/o no_ck --no-ckdata: 0m0.490s
  dbgbmap.old.py:                                  0m0.231s

Note that there's no point in adopting this in dbgbmapd3.py: 1. svg
rendering dominates (probably, I haven't measured this), and 2. we
default to showing the littlefs mount string instead of mdir/btree/data
percentages.

scripts/dbglfs.py

@@ -2723,18 +2723,6 @@ class Lfs:
         # is the filesystem corrupt?
         self.corrupt = corrupt
 
-        # check mroot
-        if not self.corrupt and not self.ckmroot():
-            self.corrupt = True
-
-        # check magic
-        if not self.corrupt and not self.ckmagic():
-            self.corrupt = True
-
-        # check gcksum
-        if not self.corrupt and not self.ckcksum():
-            self.corrupt = True
-
         # create the root directory, this is a bit of a special case
         self.root = self.Root(self)
 
@@ -2827,11 +2815,41 @@ class Lfs:
 
     @classmethod
     def fetch(cls, bd, blocks=None, trunk=None, *,
-            depth=None):
+            depth=None,
+            no_ck=False,
+            no_ckmroot=False,
+            no_ckmagic=False,
+            no_ckgcksum=False):
         # Mtree does most of the work here
         mtree = Mtree.fetch(bd, blocks, trunk,
                 depth=depth)
-        return cls(bd, mtree)
+
+        # create lfs object
+        lfs = cls(bd, mtree)
+
+        # don't check anything?
+        if no_ck:
+            return lfs
+
+        # check mroot
+        if (not no_ckmroot
+                and not lfs.corrupt
+                and not lfs.ckmroot()):
+            lfs.corrupt = True
+
+        # check magic
+        if (not no_ckmagic
+                and not lfs.corrupt
+                and not lfs.ckmagic()):
+            lfs.corrupt = True
+
+        # check gcksum
+        if (not no_ckgcksum
+                and not lfs.corrupt
+                and not lfs.ckgcksum()):
+            lfs.corrupt = True
+
+        return lfs
 
     # check that the mroot is valid
     def ckmroot(self):
@@ -2844,7 +2862,7 @@ class Lfs:
         return self.config.magic.data == b'littlefs'
 
     # check that the gcksum checks out
-    def ckcksum(self):
+    def ckgcksum(self):
         return crc32ccube(self.cksum) == int(self.gstate.gcksum)
 
     # read custom attrs
@@ -4531,7 +4549,7 @@ def main(disk, mroots=None, paths=None, *,
                         lfs.mbweightrepr(), lfs.mrweightrepr(),
                         lfs.rev,
                         lfs.cksum,
-                        '' if lfs.ckcksum() else '?'))
+                        '' if lfs.ckgcksum() else '?'))
 
         # dynamically size the id field
         w_width = max(