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

Before 14f7dd63 "[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,
c002a6c7

 "[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 <David.Woodhouse@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>