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
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