LU-14217 osd-zfs: allow SEEK_HOLE/DATA only with sync

LU-14217 osd-zfs: allow SEEK_HOLE/DATA only with sync

ZFS doesn't report valid offset for SEEK_DATA if there are dirty
data, but may report SEEK_HOLE correctly that cause unreliable
results when same offset can be reported as HOLE (correctly) and
also as DATA, incorrectly but because switching to generic approach,
assuming all file is data and hole beyond end of file.

To avoid that we have to sync dirty data when dmu_offset_next()
reports EBUSY and repeat lseek call. Considering that this can
cause slowdown this behavior is controlled via new 'sync_on_lseek'
option. With this option turned off osd-zfs reports that it doesn't
support SEEK_DATA/HOLE because we cannot use unrealiable results
in our tools to copy sparse data

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Ic92c127628ce517a9c2f79f595a1d16116930383
Reviewed-on: https://review.whamcloud.com/40970
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>