Btrfs: fix error return code in btrfs_init_test_fs()

Fix to return a negative error code from the kern_mount() error handling
case instead of 0(ret is set to 0 by register_filesystem), as done
elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>