sys/linux, tools/syz-imagegen: allow `syz_mount_image` to change directory

Add a boolean argument to the `syz_mount_image` pseudo-syscall. When
this is true, `syz_mount_image` will change directory to the mountpoint
after mounting the image passed.

Experimentation suggests that to reproduce many non-`ext4` filesystem
bugs, it is sufficient to mount the filesystem within an `ext4`-based VM
and then change directory to the mountpoint before executing code.
This change aims to increase the probability that a mount operation will
be succeeded by the corresponding change in directory, and so increase
the probability of finding non-`ext4` bugs.

We also have to update the `syz-imagegen` tool. Now it generates seed
`syz_mount_image` calls with change of directory enabled. The previous
behaviour (i.e. no change of directory) will be recovered by use of
existing corpuses and fuzzing the change-of-directory argument.

The next commit will regenerate all `syz_mount_image` seeds.

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