SEEK_DATA/SEEK_HOLE support

Add SEEK_DATA/SEEK_HOLE by creating our own zpl_file_llseek().
Tested with xfstests 285 and 286. test0[7-9] of 285 will try to reserve
the blocks first via fallocate(2), we will fail there as currently we
don't support that.

Signed-off-by: Li Dongyang <dongyang.li@anu.edu.au>
Closes #1384