fs: move I_DIRTY_INODE to fs.h

commit 0e11f6443f522f89509495b13ef1f3745640144d upstream.

And use it in a few more places rather than opencoding the values.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>