xfs: add .fileattr_set and fileattr_get callbacks for symlinks

As there are now file_getattr() and file_setattr(), xfs_quota will
call them on special files. These new syscalls call ->fileattr_get/set.

Symlink inodes don't have callbacks to set file attributes. This
patch adds them. The attribute values combinations are checked in
fileattr_set_prepare().

Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>