Complete UMH support for Linux 5.9+

[solardiz]

This issue is to track the missing bit of work on UMH mentioned by Adam in PR #18: #18 (comment)

In there, Adam wrote:

I've reviewed last kernel changes and it looks like they removed the call_usermodehelper_setup_file() function. To be able to support that case, blob_to_mnt() function is created:
https://patchwork.kernel.org/project/linux-security-module/patch/87d05mf0j9.fsf_-_@x220.int.ebiederm.org/

I think we can merge your commits as a temporary fix, however, if bpfilter dynamically loads program, it can generate FP:
https://elixir.bootlin.com/linux/latest/source/net/ipv4/bpfilter/sockopt.c#L72
https://elixir.bootlin.com/linux/latest/source/kernel/usermode_driver.c#L12

[solardiz]

Looks like a user faced this issue, but doesn't mind it: https://www.opennet.ru/openforum/vsluhforumID3/123973.html#2

[327045.342864] [p_lkrg] LKRG initialized successfully!
. . .
[422488.049526] [p_lkrg] UMH is executing file from memory...