Fix: pass private data to context callbacks

commit 4e48b5d2debaf ("Refactoring: add back constness of public API structures")
changes the type of the first argument received by context callbacks
from a struct lttng_ust_ctx_field pointer to a void private data pointer.

That commit failed to change some call sites, which were still passing
the struct lttng_ust_ctx_field pointer rather than the private data
pointer. This was not caught by compiler errors because we are loosening
from a structure pointer to a void pointer.

Fixes: #1303
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I653f454b5dc4cfcc229b3a0e17b2dc6abe9fd658