ptrace reads/writes memory using uvm_io, which generates an temporary

ptrace reads/writes memory using uvm_io, which generates an temporary
alias mapping using uvm_map_extract.  With xonly now operational, this
alias mapping is created with minprot, which for text will be xonly, and
the kernel cannot read it (unless the architecture has implied read for
exec from kernel pov).
Pass UVM_IO_FIXPROT to create the alias with maxprot instead.
ok kettenis