modpost: simplify file name generation of *.mod.c files

Avoid the variable length array (vla), just use PATH_MAX instead.
This not only makes this code clang friedly, it also leads to a
code size reduction:

   text    data     bss     dec     hex filename
  51765    2224   12416   66405   10365 scripts/mod/modpost.old
  51677    2224   12416   66317   1030d scripts/mod/modpost.new

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>