fat: relax the permission check of fat_setattr()

New chmod() allows only acceptable permission, and if not acceptable, it
returns -EPERM.  Old one allows even if it can't store permission to on
disk inode.  But it seems too strict for users.

E.g.  https://bugzilla.redhat.com/show_bug.cgi?id=449080: With new one,
rsync couldn't create the temporary file.

So, this patch allows like old one, but now it doesn't change the
permission if it can't store, and it returns 0.

Also, this patch fixes missing check.

Signed-off-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>