btrfs: Use word mathcing for _btrfs_get_subvolid()

Current _btrfs_get_subvolid() can't handle the following case at all:
  # btrfs subvol list $SCRATCH_MNT
  ID 256 gen 9 top level 5 path subv1
  ID 257 gen 7 top level 256 path subv1/subv2
  ID 258 gen 8 top level 256 path subv1/subv3
  ID 259 gen 9 top level 256 path subv1/subv4

If we call "_btrfs_get_subvolid $SCRATCH_MNT subv1" we will get a list
of all subvolumes, not the subvolid of subv1.

To address this problem, we go egrep to match $name which starts with a
space, and at the end of a line.
So that all other subvolumes won't hit.

Suggested-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

common/btrfs

@@ -7,7 +7,7 @@ _btrfs_get_subvolid()
 	mnt=$1
 	name=$2
 
-	$BTRFS_UTIL_PROG sub list $mnt | grep $name | awk '{ print $2 }'
+	$BTRFS_UTIL_PROG sub list $mnt | egrep "\s$name$" | awk '{ print $2 }'
 }
 
 # _require_btrfs_command <command> [<subcommand>|<option>]