kern: mountroot: avoid fd leak in .md parsing

parse_dir_md() opens /dev/mdctl but only closes the resulting fd on
success, not upon failure of the ioctl or when we exceed the md unit
max.

Reviewed by: kib (slightly previous version)
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
X-NetApp-PR: #62
Differential Revision: https://reviews.freebsd.org/D31229