modpost: get the *.mod file path more simply

get_src_version() strips 'o' or 'lto.o' from the end of the object file
path (so, postfixlen is 1 or 5), then adds 'mod'.

If you look at the code closely, mod->name already holds the base path
with the extension stripped.

Most of the code changes made by commit 7ac204b5

 ("modpost: lto:
strip .lto from module names") was actually unneeded.

sumversion.c does not need strends(), so it can get back local in
modpost.c again.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>