osd: describe osd ops as READ, WRITE, or READ|WRITE

osd: describe osd ops as READ, WRITE, or READ|WRITE

An op is either a pure read (read, stat), a pure (blind) write (write
extent, delete), or read+write (e.g., read/modify/write).

Writes with read portions that are captured by ProjectedObjectInfo (size,
truncate attr) can pretend to be purely write (as they are now).