xfs_db: create unlinked inodes

Create an expert-mode debugger command to create unlinked inodes.
This will hopefully aid in simulation of leaked unlinked inode handling
in the kernel and elsewhere.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>