LU-9409 llite: Add tiny write support

LU-9409 llite: Add tiny write support

If a page is already dirty in the page cache, we can write
to it without a full i/o.  This improves performance for
writes of < 1 page dramatically.

Append writes are a bit tricky, requiring us to take the
range lock (which we can normally avoid), but they are
still much faster than the normal i/o path.

Performance numbers with dd, on a VM with an older Xeon.

All numbers in MiB/s.

                8 bytes 1KiB
Without patch:  .75     75
With patch:     6.5     153

Cray-bug-id: LUS-1705
Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I75cc72ceb5f174a5394af8ffe5df4fe9583f19a3
Reviewed-on: https://review.whamcloud.com/27903
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>