Fix a bug that allowed a tracing process (e.g. gdb) to write
to a memory-mapped file in the traced process's address space
even if neither the traced process nor the tracing process had
write access to that file.

Security:	CVE-2013-2171
Security:	FreeBSD-SA-13:06.mmap
Approved by:	so

(cherry picked from commit e6d550e66f5902adc6b281537a8602bdf435a680)