This patch fixes a compiler warning:
../../../testsuites/fstests/fsdosfsname01/init.c:430:19: warning:
'%s' directive output may be truncated writing up to 6424 bytes into
a region of size 257 [-Wformat-truncation=]
The buffer 'dirname' is exactly large enough so that no truncation
can ever occur. Using the return value of snprintf() is an official
supported way to suppress the warning.
I considered the comment of Joel Sherrill about not replacing snprintf():
https://lists.rtems.org/pipermail/devel/2020-September/062113.html
0a761a58c9