Btrfs: return ENOMEM rather than use BUG_ON when btrfs_alloc_path fails

Return ENOMEM rather trigger BUG_ON, fix it.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Reviewed-by: Zach Brown <zab@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>