LU-13805 llite: Convert allocate/get to use pvec

LU-13805 llite: Convert allocate/get to use pvec

ll_allocate_dio_buffer and ll_get_user_pages both basically
work on the ll_dio_pages pvec, so this patch converts them
to do so explicitly - this makes it more obvious what
they're doing and their connection to the rest of the DIO
code.  This makes them less generic, but they have no other
users and seem unlikely to acquire any.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: If643d7684d89e0e0c81ee9d13f0d94f84ed87a56
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50168
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>