Execution of get_anon_bdev concurrently and preemptive kernel all
could bring race condition, it isn't enough to check dev against
its upper limitation with equality operator only.

This patch fix it.
Signed-off-by: NWang YanQing <udknight@gmail.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>