LU-13132 osd: osd-zfs to cache dbufs for llog objects

LU-13132 osd: osd-zfs to cache dbufs for llog objects

working set for llog objects is tiny and very predictable. osd-zfs
can cache couple dbufs (first block storing the header and last
block for new records).

for sanity/60a (llog test) it gives 5939307 hits and 5776 misses
while average osd_write() goes down from 1.09 usec to 0.27 usec,
total time for sanity/60a: before - 153s, after - 101s.

this approach can be used in few other cases like last_rcvd.

Change-Id: Icc0126658894085d33ef79ae41ac6c1ed4140f4c
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/37222
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>