generic: Add more SEEK_HOLE tests

generic: Add more SEEK_HOLE tests

Add tests for bugs found in ext4 & xfs SEEK_HOLE implementations
fixed by following patches:

xfs: Fix missed holes in SEEK_HOLE implementation
ext4: Fix SEEK_HOLE

We add tests to seek_sanity_test as it is easiest to reuse its
infrastructure for seek tests, however not to regress generic/285
which uses seek_sanity_test we don't run new tests by default.
Instead we add options to select a range of tests to run and run new
tests from this new test.

[eguan: add $tmp definition and cleanup $tmp.* on exit]

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>