common/rc: factor out _scratch_xfs_[get|set]_sb_field

It's common to get and set the values of fields in XFS super block,
so factor them out as scratch_xfs_[get|set]_sb_field, reimplement
them based on _scratch_xfs_[get|set]_metadata_field, and update the
related test cases accordingly.

Also move _scratch_xfs_[get|set]_metadata_field from common/fuzzy to
common/xfs.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/xfs/307

@@ -71,18 +71,11 @@ _set_agf_data() {
 }
 
 _get_sb_data() {
-	field="$1"
-	shift
-
-	_scratch_xfs_db -c 'sb 0' "$@" -c "p $field"  | awk '{print $3}'
+	_scratch_xfs_get_sb_field "$@"
 }
 
 _set_sb_data() {
-	field="$1"
-	value="$2"
-	shift; shift
-
-	_scratch_xfs_db -x -c 'sb 0' "$@" -c "write $field -- $value"  >> $seqres.full
+	_scratch_xfs_set_sb_field "$@" >> $seqres.full
 }
 
 _filter_leftover() {