NFSD: Replace READ* macros in nfsd4_decode_compound()

And clean-up: Now that we have removed the DECODE_TAIL macro from
nfsd4_decode_compound(), we observe that there's no benefit for
nfsd4_decode_compound() to return nfs_ok or nfserr_bad_xdr only to
have its sole caller convert those values to one or zero,
respectively. Have nfsd4_decode_compound() return 1/0 instead.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>