FreeBSD/src 1850df8 (r355417)sys/kern kern_linker.c subr_firmware.c

MFC r355108 and r355170:
Fix panic when loading kernel modules before root file system is mounted.
Make sure the rootvnode is always NULL checked.

Differential Revision:  https://reviews.freebsd.org/D22545
PR:             241639
Sponsored by:   Mellanox Technologies
DeltaFile
+6-2sys/kern/kern_linker.c
+0-6sys/kern/subr_firmware.c
+6-82 files

UnifiedSplitRaw