IMA: Add support for file reads without contents

IMA: Add support for file reads without contents

When the kernel_read_file LSM hook is called with contents=false, IMA
can appraise the file directly, without requiring a filled buffer. When
such a buffer is available, though, IMA can continue to use it instead
of forcing a double read here.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Link: https://lore.kernel.org/lkml/20200706232309.12010-10-scott.branden@broadcom.com/
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Link: https://lore.kernel.org/r/20201002173828.2099543-13-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>