Add SEEK_DATA/SEEK_HOLE to lseek()/llseek()

This patch restores the zfs_holey() function from OpenSolaris.
This was removed by commit 3558fd7 because it wasn't clear we
had a use for it in ZoL.  However, this functionality is a
prerequisite for adding SEEK_DATA/SEEK_HOLE support to the ZPL.

The approach taken was the rework zfs_holey() as little as
possible and then just wrap the code as needed to ensure
correct locking and error handling.

Tested with xfstests 285 and 286.  All tests pass except for
7-9 of 285 which try to reserve blocks first via fallocate(2)
and fail because fallocate(2) is not yet supported.

Note that the filp->f_lock spinlock did not exist prior to
Linux 2.6.30, but we avoid the need for autotools check by
virtue of the fact that SEEK_DATA/SEEK_HOLE support was not
added until Linux 3.1.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes zfsonlinux/zfs#1384