target: Fix target_core_register_fabric() for built-in fabric modules

If we try to create a fabric directory in configfs for one of the
default hard-coded fabric modules (iscsi and loopback), and that
fabric is actually built into the kernel, then the operation will
spuriously fail because request_module() (for the code that's already
linked into the kernel) fails.

Fix this by running the autoprobing code only if an initial
target_core_get_fabric() fails.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>