target: transport_subsystem_check_init cleanups

Remove the now unnecessary extra call to transport_subsystem_check_init() in
target_core_register_fabric(), and also merge transport_subsystem_reqmods()
directly into transport_subsystem_check_init().

Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>