LU-9618 clio: Use readahead for partial page write

LU-9618 clio: Use readahead for partial page write

When writing to a region of a file less than file size
(either an existing file or a shared file with multiple
writers), writes of less than one page in size must first
read in that page.

This results in extremely poor performance. For random I/O,
there's no easy improvements available, but the sequential
case can benefit enormously by using readahead to bring in
those pages.

This patch connects ll_prepare_partial_page to the readahead
infrastructure.

This does not affect random I/O or large unaligned writes,
where readahead does not detect I/O.

Benchmarks are from a small VM system, files are NOT in
cache when rewriting.

Write numbers are in MB/s.

File per process:
    access             = file-per-process
    ordering in a file = sequential offsets
    ordering inter file= no tasks offsets
    clients            = 1 (1 per node)
    repetitions        = 1
    blocksize          = 1000 MiB
    aggregate filesize = 1000 MiB

New file (best case):
xfsize  ppr	write
1KiB	n/a	59.44
5KiB	n/a	164.5

Rewrite of existing file:
xfsize  ppr	re-write
1KiB	off	4.65
1KiB	on	48.40
5KiB	off	12.95
5KiB	on	143.3

Shared file writing:
	access             = single-shared-file
	ordering in a file = sequential offsets
	ordering inter file= no tasks offsets
	clients            = 4 (4 per node)
	repetitions        = 1
	blocksize          = 1000 MiB
        aggregate filesize = 4000 MiB

xfsize  ppr     write
1KiB	off	11.26
1KiB	on	58.72
5KiB	off	18.7
5KiB	on	127.3

Cray-bug-id: LUS-188
Signed-off-by: Patrick Farrell <paf@cray.com>
Signed-off-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Change-Id: I822395995ee23b1c9ca289ae982e5294b69a0cff
Reviewed-on: https://review.whamcloud.com/27544
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>