LU-10683 osd_zfs: set offset in page correctly

In osd_bufs_get_write, the offset in the first page should
be calculated on the offset parameter instead of zero.

Change-Id: I6592d8b5b0162b92953d59e2662a4381ba3e89ba
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/32788


Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>