NFSD: Count bytes instead of pages in the NFSv2 READDIR encoder

[ Upstream commit 8141d6a2 ]

Clean up: Counting the bytes used by each returned directory entry
seems less brittle to me than trying to measure consumed pages after
the fact.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>