LU-10810 osd: implement lseek method in OSD

LU-10810 osd: implement lseek method in OSD

Introduce lseek support in dt_object methods and at OSD level

This is server support for SEEK_HOLE and SEEK_DATA distributed
request, it returns a resulting offset or error code

ZFS support is added as well but it is not yet available due to
missing export of dmu_offset_next() function.

Patch adds also osd_quasi_file() common method to create fake
file structure to use in inode i_fop calls

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I3802743e31c94d2de219b72a540c5df9c72f1897
Reviewed-on: https://review.whamcloud.com/39706
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>