kpatch-build: replace special characters in module name

Having a '.' in the kmod name confuses lsmod, which prints "Size" and
"Used by" values of -2.  Prevent any special characters other than '_'
and '-', so that our patch module names will be consistent with typical
kmod names.
Merge pull request #170 from jpoimboe/module-special-chars

kpatch-build: replace special characters in module name