target: Drop bogus ERR_PTR usage in target_fabric_configfs_init

In the original code, there were several places inside the
target_fabric_configfs_init() function that returned NULL on error
and one place the returned an ERR_PTR.  There are two places that
call this function and they only check for NULL returns; they don't
check for ERR_PTRs.  So I've changed the ERR_PTR so now the function
only returns NULL on error.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>