SUNRPC: convert RPC_TASK_* constants to enum

The RPC_TASK_* constants are defined as macros, which means that most
kernel builds will not contain their definitions in the debuginfo.
However, it's quite useful for debuggers to be able to view the task
state constant and interpret it correctly. Conversion to an enum will
ensure the constants are present in debuginfo and can be interpreted by
debuggers without needing to hard-code them and track their changes.

Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>