BUG/MINOR: debug: properly mark the entire libs archive read-only

BUG/MINOR: debug: properly mark the entire libs archive read-only

In 3.4-dev7, commit e1738b665d ("MINOR: debug: read all libs in memory
when set-dumpable=libs") reads dependencies into memory to store them as
a tar archive for later debugging. There was an attempt to mark the whole
archive read-only, except that the size passed in argument to mprotect()
is wrong: lib_size is only assigned after the operation and is still zero
at the moment this is done. new_size ought to be used instead.

This needs to be backported wherever the commit above is backported, at
least 3.2.