Introduce IO_NOMACCHECK, a flag that will be passed to vn_rdwr() to

Introduce IO_NOMACCHECK, a flag that will be passed to vn_rdwr() to
indicate that the calling code has already performed necessary MAC
checks (if any) for this operation.  This flag will help resolve
layering problems that existing because vn_rdwr() is called both
on behalf of user processes directly (such as in system calls of
various sorts, during core dumps, etc), as well as deep in the file
system code on behalf of the file system (such as in UFS, ext2fs,
etc).  Code that is acting on behalf of a kernel service rather
than explicitly on behalf of a user process will specify this flag.
By default, MAC checks will be performed (and generally should
be performed).

Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, NAI Labs