cifs: implement get acl method

cifs: implement get acl method

The current way of setting and getting posix acls through the generic
xattr interface is error prone and type unsafe. The vfs needs to
interpret and fixup posix acls before storing or reporting it to
userspace. Various hacks exist to make this work. The code is hard to
understand and difficult to maintain in it's current form. Instead of
making this work by hacking posix acls through xattr handlers we are
building a dedicated posix acl api around the get and set inode
operations. This removes a lot of hackiness and makes the codepaths
easier to maintain. A lot of background can be found in [1].

In order to build a type safe posix api around get and set acl we need
all filesystem to implement get and set acl.

So far cifs wasn't able to implement get and set acl inode operations
because it needs access to the dentry. Now that we extended the set acl
inode operation to take a dentry argument and added a new get acl inode
operation that takes a dentry argument we can let cifs implement get and
set acl inode operations.

This is mostly a copy and paste of the codepaths currently used in cifs'
posix acl xattr handler. After we have fully implemented the posix acl
api and switched the vfs over to it, the cifs specific posix acl xattr
handler and associated code will be removed and the code duplication
will go away.

Note, until the vfs has been switched to the new posix acl api this
patch is a non-functional change.

Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>