vfs: add i_op->dentry_open()

Add a new inode operation i_op->dentry_open().  This is for stacked filesystems
that want to return a struct file from a different filesystem.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

diff --cc Documentation/filesystems/Locking
Simple merge

diff --cc Documentation/filesystems/vfs.txt
Simple merge

diff --cc fs/namei.c
Simple merge

diff --cc fs/open.c
Simple merge

diff --cc include/linux/fs.h
Simple merge