ARM64 readahead: fault retry breaks mmap file read random detection

Description from commit 45cac65b
    ("readahead: fault retry breaks mmap file read random detection")

.fault now can retry.  The retry can break state machine of .fault.  In
filemap_fault, if page is miss, ra->mmap_miss is increased.  In the second
try, since the page is in page cache now, ra->mmap_miss is decreased.  And
these are done in one fault, so we can't detect random mmap file access.

Add a new flag to indicate .fault is tried once.  In the second try, skip
ra->mmap_miss decreasing.  The filemap_fault state machine is ok with it.

I only tested x86, didn't test other archs, but looks the change for other
archs is obvious, but who knows :)

< snip >

Yup, arm64 needs this too! Random read improves by 250%, sequential
read improves by 40%, and random write by 400% to an eMMC device with
dm crypto wrapped around it.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Signed-off-by: Riley Andrews <riandrews@android.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Shaohua Li <shaohua.li@fusionio.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Bug: 23181629
Bug: 23385923
Change-Id: Ia4de1199164d6b5d4430f5518daf2aa5a71a4059