fat: expand a slightly out-of-date comment

fat: expand a slightly out-of-date comment

The file namei.c seems to have been renamed to namei_msdos.c, so I decided
to update the comment with the correct name, and expand it a bit to tell
the reader what to look for.

Link: http://lkml.kernel.org/r/20180928194947.23932-1-mihir@cs.utexas.edu
Signed-off-by: Mihir Mehta <mihir@cs.utexas.edu>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>