BKL: Remove BKL from squashfs

The BKL is only used in put_super and fill_super, which are both protected
by the superblocks s_umount rw_semaphore. Therefore it is safe to remove
the BKL entirely.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Phillip Lougher <phillip@lougher.demon.co.uk>