xfs: remove the kuid/kgid conversion wrappers

xfs: remove the kuid/kgid conversion wrappers

Remove the XFS wrappers for converting from and to the kuid/kgid types.
Mostly this means switching to VFS i_{u,g}id_{read,write} helpers, but
in a few spots the calls to the conversion functions is open coded.
To match the use of sb->s_user_ns in the helpers and other file systems,
sb->s_user_ns is also used in the quota code.  The ACL code already does
the conversion in a grotty layering violation in the VFS xattr code,
so it keeps using init_user_ns for the identity mapping.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>