cls/rgw: duplicate reshard checks in all cls_rgw write operations

cls/rgw: duplicate reshard checks in all cls_rgw write operations

with the addition of the reshard log, all write ops now have to read the
omap header themselves. this read duplicates the one in
cls_rgw_guard_bucket_resharding(), leading to a minor performance
regression

this commit prepares a long-term fix for this regression by duplicating
the the reshard check done by cls_rgw_guard_bucket_resharding() in each
write operation. this will allow the cls_rgw_guard_bucket_resharding()
calls to be removed from rgw two releases later when we can guarantee
that all OSDs are performing this check for all write operations

Signed-off-by: Casey Bodley <cbodley@redhat.com>