This patch makes three small changes to nfsd_readdir().

First, the 'filldir' routine for NFSv4 may return an arbitrary error,
which should become the return value for nfsd_readdir().  I implemented
this by adding an 'nfserr' field to the 'struct readdir_cd'.

Second, in NFSv4 the caller of nfsd_readdir() will specify an attribute
bitmap, which must be communicated to the 'filldir' routine.  I implemented
this by adding a @bitmap parameter to nfsd_readdir() and a corresponding
field in the 'struct readdir_cd'.  (The bitmap is not interpreted in any
way by nfsd_readdir().)

Finally, NFSv4 defines a new error nfserr_readdir_nospc, which indicates
that there was not enough buffer space to encode a single entry.