lindebugfs: check that name is set as otherwise pfs_alloc_node_flags() panics

[bz]

Summary

I have hit the case multiple times that some LinuxKPI field may not be
set during driver bringup and lindebugfs would cause a panic.
The backtrace goes like:

strlen() at strlen+0x54
pfs_create_dir() at pfs_create_dir+0x41
debugfs_create_dir() at debugfs_create_dir+0xa1
...

We likely should at least add an assert to pfs_create_dir() if
name is NULL like we have for pfs_add_node() but for lindebugfs at
least make this a graceful error and continue without creating
the dir instead of panicing.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days

[kevans]

I have some more pseudofs patches in the pipeline still, I'll look at being consistent there as follow-up