modpost: Fix resource leak in read_dump()

Function read_dump() memory maps the input via grab_file(), but fails to call
the corresponding unmap function. Add the missing call to release_file().
Detected by Coverity: CID 1192419

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>