aufs: show the branch id via sysfs

It is mainly for a new feature move-down.
The feature is implemented in aufs3.9 and later actually, and this
commit is useless for the branches before aufs3.9.
But it is good to keep the consistency of interface.

Conflicts:
	fs/aufs/branch.h

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
aufs: for pmfsrr, convert a param into a flag

The additional parameter for au_wbr_create_operations->create()
specifies whether the dentry corresponds to a dir or not.
In order to introduce a new policy pmfsrr, aufs needs another info.
So convert the pamameter into flags to be able to represent multiple
info.

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
aufs: for pmfsrr, add PARENT flag

When the flag is set, au_mfs() searches the branch which is
"most-free-space AND the parent exists."

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
aufs: pmfsrr

option syntax: create=pmfsrr:low:[second]

Selects a writable branch where the parent dir exists, such as tdp mode.
When the parent dir exists on multiple writable branches, aufs selects the
one which has most free space, such as mfs mode.
If there are less than 'low' bytes available on all branches where the
parent dir exists, aufs selects the one which has the most free space
regardless the parent dir.

Suggested-by: Michael Johnson - MJ <mj@revmj.com>
Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
Merge remote-tracking branch 'aufs3-standalone/aufs3.2/00base' into aufs3.2/10static
Merge branch 'aufs3.2/10static' into aufs3.2/11proc_map
Merge branch 'aufs3.2/11proc_map' into aufs3.2/18public
aufs: bugfix, support an error in copy-up

By implementing a new feature "move-down", an error which have never
happened can rise in copy-up (actually copy-down).
So add an error-handling code.

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
Merge remote-tracking branch 'aufs3-standalone/aufs3.2/00base' into aufs3.2/10static
Merge branch 'aufs3.2/10static' into aufs3.2/11proc_map
Merge branch 'aufs3.2/11proc_map' into aufs3.2/18public