module: fold usermode helper kmod into modules directory

The kernel/kmod.c is already only built if we enabled modules, so
just stuff it under kernel/module/kmod.c and unify the MAINTAINERS
file for it.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

kernel/module/Makefile

@@ -7,7 +7,9 @@
 # and produce insane amounts of uninteresting coverage.
 KCOV_INSTRUMENT_module.o := n
 
-obj-y += main.o strict_rwx.o
+obj-y += main.o
+obj-y += strict_rwx.o
+obj-y += kmod.o
 obj-$(CONFIG_MODULE_DECOMPRESS) += decompress.o
 obj-$(CONFIG_MODULE_SIG) += signing.o
 obj-$(CONFIG_LIVEPATCH) += livepatch.o