IB/core: Fix deadlock on uverbs modify_qp error flow

The deadlock occurs in __uverbs_modify_qp: we take a lock (idr_read_qp)
and in case of failure in ib_resolve_eth_l2_attrs we don't release
it (put_qp_read).  Fix that.

Fixes: ed4c54e5

 ("IB/core: Resolve Ethernet L2 addresses when modifying QP")
Signed-off-by: Moshe Lazer <moshel@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>