nfs: fix failure to parse EXCHANGE_ID arguments

The exchange_id spo_must_enforce/spo_must_allow bitmaps are exceeding
this arbitrary MAX_BITMAPS constant, causing EXCHANGE_ID parsing to
fail.

Even in the case of attributes we may need more than 4 words soon, even
if we don't I'm not sure if it's actually illegal to zero-pad them out
to something longer, and even if that is illegal I don't think it's
really helpful of wireshark to stop parsing.

Keep the constant just as a sanity check but make it really large.

Ping-Bug: 10649
Change-Id: I3dff3ebde2d7f74f8cbe60a92c853463da66ae6a
Reviewed-on: https://code.wireshark.org/review/5144
Reviewed-by: Evan Huus <eapache@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>