nfsd: ensure nfsd_file_do_acquire() does not use a non-opened file

In the Linux kernel, the following vulnerability has been resolved:

nfsd: ensure nfsd_file_do_acquire() does not use a non-opened file

->atomic_open is permitted to return success without actually opening
the file. It indicates this by calling finish_no_open().
This means dentry_create() can return a file which hasn't been opened.
This is extremely unlikely as ->atomic_open handlers typically
use finish_no_open() only for already existing files, and dentry_create()
isn't called in that case, and the parent being locked should prevent
races.

However out of an abundance of caution it seems wise to teach nfsd to
only use the file returned by dentry_create() if FMODE_OPENED is set,
indicating that it has in fact been opened.