ima: limit the number of ToMToU integrity violations

Each time a file in policy, that is already opened for read, is opened
for write, a Time-of-Measure-Time-of-Use (ToMToU) integrity violation
audit message is emitted and a violation record is added to the IMA
measurement list.  This occurs even if a ToMToU violation has already
been recorded.

Limit the number of ToMToU integrity violations per file open for read.

Note: The IMA_MAY_EMIT_TOMTOU atomic flag must be set from the reader
side based on policy.  This may result in a per file open for read
ToMToU violation.

Since IMA_MUST_MEASURE is only used for violations, rename the atomic
IMA_MUST_MEASURE flag to IMA_MAY_EMIT_TOMTOU.

Cc: stable@vger.kernel.org # applies cleanly up to linux-6.6
Tested-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Tested-by: Petr Vorel <pvorel@suse.cz>
Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>