LU-14918 osd: don't declare similar zfs writes twice

LU-14918 osd: don't declare similar zfs writes twice

in some cases (like overstriping) the same operations can be
declared multiple times (new llog records) and this lead to
huge number of credits and performance degradation. we can
avoid this checking for duplicate declarations.
notice each declare operation results in a allocation in ZFS.

the example for an overstriped file (2000 stripes over 4 OSTs),
declare ops			before	after
create:				2001	2
unlink:				10001	10

creation of 1K-stripe files (over 4 OSTs) is 2.5% faster.
removal of 1K-stripe files is 44% faster.

single-stripe file creation/removal does not degrade.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I5d9e6d3a1574ccd7bf97fd3a67ab4fff0b6a352c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49701
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>