fat: fix fake_offset handling on error path

fat: fix fake_offset handling on error path

For the root directory, .  and ..  are faked (using dir_emit_dots()) and
ctx->pos is reset from 2 to 0.

A corrupted root directory could cause fat_get_entry() to fail, but
->iterate() (fat_readdir()) reports progress to the VFS (with ctx->pos
rewound to 0), so any following calls to ->iterate() continue to return
the same entries again and again.

The result is that userspace will never see the end of the directory,
causing e.g.  'ls' to hang in a getdents() loop.

[[email protected]: cleanup and make sure to correct fake_offset]
Reported-by: Vegard Nossum <[email protected]>
Tested-by: Vegard Nossum <[email protected]>
Signed-off-by: Richard Weinberger <[email protected]>
Signed-off-by: OGAWA Hirofumi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>