LU-4078 mdc: fix bad ERR_PTR usage in mdc_locks.c

LU-4078 mdc: fix bad ERR_PTR usage in mdc_locks.c

In mdc_intent_open_pack() return an ERR_PTR() rather than NULL when
ldlm_prep_enqueue_req() fails. In mdc_intent_getattr_async() check the
return value of mdc_intent_getattr_pack() using IS_ERR(). Clean up the
includes in mdc_locks.c.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I89348389ccd102d1226caf16b315ac9b2a96b307
Reviewed-on: http://review.whamcloud.com/7886
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>