[SCSI] fix up scsi_eh_lock_door()

The Documentation is incorrect (we removed some functions referred to), and
none of the bug warnings now apply.  Additionally remove the spurious check on
the return from blk_get_request() which can't fail if __GFP_WAIT is passed in.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>