xfs: Make xfs_vn_rename compliant with renameat2() syscall

To be able to support RENAME_EXCHANGE flag from renameat2() system
call, XFS must have its inode_operations updated, exporting .rename2
method, instead of .rename.

This patch just replaces the (now old) .rename method by .rename2,
using the same infra-structure, but checking rename flags.  Calls to
.rename2 using RENAME_EXCHANGE flag, although now handled inside
XFS, still return -EINVAL.

RENAME_NOREPLACE is handled via VFS and we don't need to care about
it inside xfs_vn_rename.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>