avoid extra path_get/path_put cycle in path_openat()

Once we'd opened the file, nd->path and file->f_path have the
same contents.  Rather than having both pinned and nd->path
dropped by terminate_walk(), let's have them share the
references from the moment when FMODE_OPENED is set and
clear nd->path just before the terminate_walk() in such case.

To do that, we
	* add a variant of vfs_open() that does *not* do conditional
path_get() (vfs_open_borrow()); use it in do_open().
	* don't grab f->f_path.mnt in finish_open() - only
f->f_path.dentry.  Have atomic_open() drop the child dentry
in FMODE_OPENED case and return f->path.dentry without grabbing it.
	* adjust vfs_tmpfile() for finish_open() change (it
is called from ->tmpfile() instances).
	* make do_o_path() use vfs_open_borrow(), collapse path_put()
there with the conditional path_get() we would've get in vfs_open().
	* in FMODE_OPENED case clear nd->path before calling
terminate_walk().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>