vfat: Fix vfat_find() error path in vfat_lookup()

Current vfat_lookup() creates negetive dentry blindly if vfat_find()
returned a error. It's wrong. If the error isn't -ENOENT, just return
error.

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>