fuse: break up fuse_open_common()

fuse_open_common() has a lot of code relevant only for regular files and
O_TRUNC in particular.

Copy the little bit of remaining code into fuse_dir_open() and stop using
this common helper for directory open.

Also split out fuse_dir_finish_open() from fuse_finish_open() before we add
inode io modes to fuse_finish_open().

Suggested-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>