Busy the mount point which is the owner of the audit vnode, around

Busy the mount point which is the owner of the audit vnode, around
audit_record_write().  This is important so that VFS_STATFS() is not
done on the NULL or freed mp and the check for free space is
consistent with the vnode used for write.

Add vn_start_write() braces around VOP_FSYNC() calls on the audit vnode.

Move repeated code to fsync vnode and panic to the helper
audit_worker_sync_vp().

Reviewed by:	rwatson
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week