IB/uverbs: Set correct user handle for user SRQs

Store away the user handle passed in from userspace when creating an
SRQ, so that the kernel can return the correct handle when an SRQ
asynchronous event occurs.  (A 0 was incorrectly stored as the user
handle as part of the changes in 9ead190b, "IB/uverbs: Don't serialize
with ib_uverbs_idr_mutex")

Signed-off-by: Roland Dreier <rolandd@cisco.com>