md: Add MD_MUSTDEALLOC support

This adds an option to detect if hole-punching is implemented by the
underlying file system. If this flag is set, and if the underlying file
system does not support hole-punching, md(4) fails BIO_DELETE requests
with EOPNOTSUPP.

Sponsored by: The FreeBSD Foundation
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D31883