xfs: Refactor xfs_attr_set_shortform

xfs: Refactor xfs_attr_set_shortform

This patch is actually the combination of patches from the previous
version (v18).  Initially patch 3 hoisted xfs_attr_set_shortform, and
the next added the helper xfs_attr_set_fmt. xfs_attr_set_fmt is similar
the old xfs_attr_set_shortform. It returns 0 when the attr has been set
and no further action is needed. It returns -EAGAIN when shortform has
been transformed to leaf, and the calling function should proceed the
set the attr in leaf form.

Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>