IB/core, cma: Make __attribute_const__ declarations sparse-friendly

IB/core, cma: Make __attribute_const__ declarations sparse-friendly

Move the __attribute_const__ declarations such that sparse understands
that these apply to the function itself and not to the return type.
This avoids that sparse reports error messages like the following:

drivers/infiniband/core/verbs.c:73:12: error: symbol 'ib_event_msg' redeclared with different type (originally declared at include/rdma/ib_verbs.h:470) - different modifiers

Fixes: 2b1b5b601230 ("IB/core, cma: Nice log-friendly string helpers")
Signed-off-by: Bart Van Assche <[email protected]>
Cc: Sagi Grimberg <[email protected]>
Reviewed-by: Sagi Grimberg <[email protected]>
Signed-off-by: Doug Ledford <[email protected]>