Btrfs: fix __btrfs_start_workers retval

__btrfs_start_workers returns 0 in case it raced with
btrfs_stop_workers and lost the race.  This is wrong because worker in
this case is not allowed to start and is in fact destroyed.  Return
-EINVAL instead.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>