common: factor out xfs unique part from _filter_mkfs

common: factor out xfs unique part from _filter_mkfs

Most of the code in the function _filter_mkfs is xfs unique. This is
misleading that the function would be dedicated for xfs. Clean up the
function by factoring out xfs unique part to _xfs_filter_mkfs in
common/xfs. While at the same time, fix indent in _xfs_filter_mkfs to be
consistent with other functions in common/xfs.

Suggested-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>