nfsd: fix failure to set eof in readdir in some situations

nfsd: fix failure to set eof in readdir in some situations

Before 14f7dd632011bb89c035722edd6ea0d90ca6b078 "[PATCH] Copy XFS
readdir hack into nfsd code", readdir_cd->err was reset to eof before
each call to vfs_readdir; afterwards, it is set only once.  Similarly,
c002a6c7977320f95b5edede5ce4e0eeecf291ff "[PATCH] Optimise NFS readdir
hack slightly", can cause us to exit without nfserr_eof set.  Fix this.

This ensures the "eof" bit is set when needed in readdir replies.  (The
particular case I saw was an nfsv4 readdir of an empty directory, which
returned with no entries (the protocol requires "." and ".." to be
filtered out), but with eof unset.)

Cc: David Woodhouse <[email protected]>
Cc: Al Viro <[email protected]>
Signed-off-by: J. Bruce Fields <[email protected]>