new helper: audit_file()

... for situations when we don't have any candidate in pathnames - basically,
in descriptor-based syscalls.

[Folded the build fix for !CONFIG_AUDITSYSCALL configs from Chen Gang]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc fs/open.c
Simple merge

diff --cc fs/xattr.c
Simple merge

diff --cc include/linux/audit.h
Simple merge

diff --cc ipc/mqueue.c
Simple merge

diff --cc kernel/auditsc.c
Simple merge