LinuxKPI,lindebugfs: add u8 base type and blob support

Add debugfs_create_u8() based on other already present implementations.
Add a read-only implementation for debugfs_create_blob().

Both are needed for iwlwifi debugfs support.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
OKed by: jfree (earlier version)
Differential Revision: https://reviews.freebsd.org/D37090