nfsd4: filehandle leak or error exit from fh_compose()

A number of callers (nfsd4_encode_fattr(), at least) don't bother to
release the filehandle returned to fh_compose() if fh_compose() returns
an error.  So, modify fh_compose() to release the filehandle before
returning an error.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>