ima: prevent a file already mmap'ed write to be mmap'ed execute

The kernel calls deny_write_access() to prevent a file already opened
for write from being executed and also prevents files being executed
from being opened for write.  For some reason this does not extend to
files being mmap'ed execute.

From an IMA perspective, measuring/appraising the integrity of a file
being mmap'ed shared execute, without first making sure the file cannot
be modified, makes no sense.  This patch prevents files, in policy,
already mmap'ed shared write, from being mmap'ed execute.

Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>