NFSD: Add a streaming directory reader

nfsd_readdir() reads a whole directory in one call, invoking a filldir
actor for every entry.  A consumer that wants to read entries while it
encodes the reply -- as NFSv4 READDIR does, and as NFSv2 READDIR wants
to -- cannot pause the loop between entries; it can only stop by
returning nonzero from the actor and reporting eof or a full reply
back through readdir_cd.

Factor the directory read into a pull iterator.  nfsd_readdir_open()
opens the directory and seeks to the start cookie, nfsd_readdir_next()
yields one buffered entry at a time and refills from the filesystem as
needed, and nfsd_readdir_close() releases the open file.  The reader
holds the file across calls, so a consumer can pull entries during
reply encoding and stop whenever its reply buffer fills.

The page-buffer-per-batch design is retained: iterate_dir() fills a
page through the buffering actor, and entries are handed to the
consumer outside that call, so a consumer that calls ->lookup() (v3
readdirplus) still never reenters iterate_dir().

Reimplement nfsd_readdir() as a thin wrapper over the iterator, so
NFSv3 and NFSv4 keep their existing behavior; a new caller uses the
open/next/close interface directly.

Acked-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260916162859.2051-28-cel@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

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