LU-6142 mdc: minor function cleanups.

LU-6142 mdc: minor function cleanups.

mdc_getxattr_pack() doesn't exist, so remove declaration

mdc_get_lustre_md(), mdc_free_lustre_md(), and
mdc_clear_open_replay_data() are local to mdc_request.c so make them
static and remove declaration from mdc_interal.h.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ie925585f51f280a4ad4e3ae7b488dc4166fdaa8d
Reviewed-on: https://review.whamcloud.com/39738
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>