Fix: lttng-logger get_user_pages_fast error handling

Comparing a signed return value against an unsigned nr_pages performs
the comparison as "unsigned", and therefore mistakenly considers
get_user_pages_fast() errors as success.

By passing an invalid pointer to write() to the /proc/lttng-logger
interface, unprivileged user-space processes can trigger a kernel OOPS.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>