pnfs-obj: Return PNFS_NOT_ATTEMPTED in case of read/write_pagelist

objlayout driver was always returning PNFS_ATTEMPTED from it's
read/write_pagelist operations. Even on error. Fix that.

Start by establishing an error return API from io-engine, by
not returning ssize_t (length-or-error) but returning "int"
0=OK, 0>Error. And clean up all return types in io-engine.

Then if io-engine returned error return PNFS_NOT_ATTEMPTED
to generic layer. (With a dprint)

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>