kmod: build: cure compiler warnings showing up externally
kmod: build: cure compiler warnings showing up externally

When building a C source file with gcc-7 -Wshift-overflow=2, this warning
springs up:

libkmod.h: warning: result of "1 << 31" requires 33 bits to
represent, but "int" only has 32 bits [-Wshift-overflow=]

Change the two _KMOD_* identifiers to fit into 32 bits.

lucasdemarchi/kmod,lucasdemarchi/kmod,lucasdemarchi/kmod