mkfs: pass a custom cowextsize into the created filesystem

mkfs: pass a custom cowextsize into the created filesystem

Create a -d option to mkfs.xfs that enables administrators to set
the CoW extent size hint on the created files.

[sandeen: Note, the switch to xfs_flags2diflags looks like
a bugfix, but it's not - the flags set by mkfs up to this
point just happened to line up without any translation.]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>