NFSv4: Fix potential Oops in decode_op_map()

The return value of xdr_inline_decode() is not being checked, leading to
a potential Oops. Just replace the open coded array decode with the
generic XDR version.

Reported-by:  <rtm@csail.mit.edu>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>