Fix: cmd_rotate_session() returns unexpected error codes

Certain paths in cmd_rotate_session() result in its return value not
obeying the convention for commands: return LTTNG_OK on success, and a
negative LTTNG_ERR_* code on error.

This patch separates the use of 'int ret' from a separate 'enum
lttng_error_code' value to ensure 'ret' values never bubble-up to the
caller.

Note that this patch assumes that ust_app_rotate_session() returns an
lttng_error_code, which is not the case right now (upcoming patch).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>