fs/anon_inode: Introduce a new lib function anon_inode_getfile_private()

fs/anon_inode: Introduce a new lib function anon_inode_getfile_private()

Introduce a new lib function anon_inode_getfile_private(), it creates a new file
instance by hooking it up to an anonymous inode, and a dentry that describe the
"class" of the file, similar to anon_inode_getfile(), but each file holds a
single inode. Furthermore, anyone who wants to create a private anon file will
benefit from this change.

Signed-off-by: Gu Zheng <[email protected]>
Signed-off-by: Benjamin LaHaise <[email protected]>