block: respect error code from bdrv_getlength in handle_aiocb_write_zeroes

block: respect error code from bdrv_getlength in handle_aiocb_write_zeroes

Original idea beyond the code in question was the following: we have failed
to write zeroes with fallocate(FALLOC_FL_ZERO_RANGE) as the simplest
approach and via fallocate(FALLOC_FL_PUNCH_HOLE)/fallocate(0). We have the
only chance now: if the request comes beyond end of the file. Thus we
should calculate file length and respect the error code from that op.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Markus Armbruster <armbru@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>