LU-9778 llite: Read ahead should return pages read

LU-9778 llite: Read ahead should return pages read

ll_read_ahead_pages was modified by:
LU-7990 clio: revise readahead to support 16MB IO
d8467ab8a2ca15fbbd5be3429c9cf9ceb0fa78b8

And returning the count of pages read was removed.

This only affects debug, but it's very nice to have it
printed out, and several messages still try to print out
pages read ahead, but print 0.

Restore this functionality.

Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I80fe66b5195629e0c46d5d19c76e3bcc0030a22a
Reviewed-on: https://review.whamcloud.com/28052
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>