LU-15800 ofd: take a read lock for fallocate

LU-15800 ofd: take a read lock for fallocate

there is no need to take an write (exclusive) object's
lock for fallocate - we just need to serialize fallocate
vs destroy, all internal structures should be protected
by OSD and disk filesystem like the write path does.

Fixes: cdaaa87f6b ("LU-14214 ofd: fix locking in ofd_object_fallocate()")
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I65986745865ee329c5257a7efca5e79403830608
Reviewed-on: https://review.whamcloud.com/47268
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>