Fix a bug that allowed a tracing process (e.g. gdb) to write

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

git-svn-id: svn://svn.freebsd.org/base/stable/9@251902 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f