Avoid strict DM interface version dependencies (#5519)

Compiling against the dm-ioctl.h header as provided by the Linux kernel
will embed the DM interface version number. Running an older kernel can
result in an error like this on shutdown:

Could not detach DM dm-11: ioctl mismatch, kernel(4.34.4), user(4.35.4)

Work around this by shipping a local copy of dm-ioctl.h. We need at
least the version from 3.13 for DM_DEFERRED_REMOVE [1], so bump the
requirements in README accordingly.

[1] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=2c140a246dc0bc085b98eddde978060fcec1080c

Fixes: #5492