integrity: move integrity_audit_msg()

This patch moves the integrity_audit_msg() function and defintion to
security/integrity/, the parent directory, renames the 'ima_audit'
boot command line option to 'integrity_audit', and fixes the Kconfig
help text to reflect the actual code.

Changelog:
- Fixed ifdef inclusion of integrity_audit_msg() (Fengguang Wu)

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