Replace md(4) usage in diskless(8) script rc.initdiskless with tmpfs(5).
Need to multiply the size of the disk passed to mount_md by 512 as mdmfs
expects number of 512-byte blocks while tmpfs size option wants number of
bytes.

Reviewed by: brooks
Approved by: sjg (mentor)
Obtained from: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D11106