xfs: constify the name argument to various directory functions

Various directory functions do not modify their @name parameter,
so mark it const to make that clear.  This will enable us to mark
the global xfs_name_dotdot variable as const to prevent mischief.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>