modpost: simplify file name generation of *.mod.c files
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 <64568d9183dcfcdd0a6fb47964820a1cdaf94ecc@googlemail.com>
Signed-off-by: Rusty Russell <df9728c9e5104131c08c7adb03af425394842596@rustcorp.com.au>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs