modpost: free allocated memory

valgrind complains that memory is not freed after allocation
with realloc() called from main() and write_dump().

So let us free the allocated memory properly.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Link: http://lkml.kernel.org/r/1470166981-6461-1-git-send-email-xypron.glpk@gmx.de


Signed-off-by: Jessica Yu <jeyu@redhat.com>