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.

Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
X-NetApp-PR: #62

(cherry picked from commit 23ecfa9d5bc4f04eb58e26018c2d15f032d5d742)