LU-15274 llite: whole file read fixes

LU-15274 llite: whole file read fixes

There are two significant issues with whole file read.

1. Whole file read does not interact correctly with fast
reads - specifically, whole file read is not recognized by
the fast read code so files below the
"max_read_ahead_whole_mb" limit will not use fast reads.
This has a significant performance impact.

2. Whole file read does not start from the beginning of the
file, it starts from the current IO index.  This causes
issues with unusual IO patterns, and can also confuse
readahead more generally (I admit to not fully understanding
what happens here, but the change is reasonable regardless.)
This is particularly important for cases where the read
doesn't start at the beginning of the file but still reads
the whole file (eg, random or backwards reads).

Performance data:
max_read_ahead_whole_mb defaults to 64 MiB, so a 64 MiB
file is read with whole file, and a 65 MiB file is not.

Without this fix:
rm -f file
truncate -s 64M file
dd if=file bs=4K of=/dev/null
67108864 bytes (67 MB, 64 MiB) copied, 7.40127 s, 9.1 MB/s

rm -f file
truncate -s 65M file
dd if=file bs=4K of=/dev/null
68157440 bytes (68 MB, 65 MiB) copied, 0.0932216 s, 630 MB/s

Whole file readahead: 9.1 MB/s
Non whole file readahead: 630 MB/s

With this fix (same test as above):
Whole file readahead: 994 MB/s
Non whole file readahead: 630 MB/s (unchanged)

Fixes: 7864a68 ("LU-12043 llite,readahead: don't always use max RPC size")
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I72f0b58e289e83a2f2a3868ef0d433a50889d4c0
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54011
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Shuichi Ihara <sihara@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>