selftests/ublk: add filesystem fio verify test for shmem_zc

selftests/ublk: add filesystem fio verify test for shmem_zc

Add test_shmemzc_03.sh which exercises shmem_zc through the full
filesystem stack: mkfs ext4 on the ublk device, mount it, then run
fio verify on a file inside the filesystem with --mem=mmaphuge.

Extend _mkfs_mount_test() to accept an optional command that runs
between mount and umount. The function cd's into the mount directory
so the command can use relative file paths. Existing callers that
pass only the device are unaffected.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://patch.msgid.link/20260331153207.3635125-10-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>