generic: Add test for seekdir

generic: Add test for seekdir

Add test checking functionality of seekdir. We check whether seekdir
gets us back to the directory entry it should and also whether
seeking to random positions does not crash the filesystem.

Unlike test generic/310 which also tests seeking, this test checks both
glibc readdir() function as well as getdents64() syscall directly. This
is because glibc readdir() implementation does a lot of caching and
processing internally thus hiding kernel from some possible problems.
Also test wider range of random offsets to have better chance of
hitting out of bound accesses or other bugs.

This is a regression test for a48fc69fe658 ("udf: Fix crash after
seekdir")

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