mon/MonCap: avoid typedef for mon_rwxa_t

This conflicts later because we have an operator<< for __u8 in OSDCap, too.
That one also needs to be similarly converted.

Signed-off-by: Sage Weil <sage@inktank.com>
osd/OSDCap: avoid typedef for rwxa_t; rename to osd_rwxa_t

Avoid implementing an operator<< on __u8, and make the naming less likely
to conflict with others.

We did a similar thing with MonCap.

Signed-off-by: Sage Weil <sage@inktank.com>