IB/uverbs: Fix an potential error pointer dereference

IB/uverbs: Fix an potential error pointer dereference

smatch reports the warning below:
drivers/infiniband/core/uverbs_std_types_counters.c:110
ib_uverbs_handler_UVERBS_METHOD_COUNTERS_READ() error: 'uattr'
dereferencing possible ERR_PTR()

The return value of uattr maybe ERR_PTR(-ENOENT), fix this by checking
the value of uattr before using it.

Fixes: ebb6796bd397 ("IB/uverbs: Add read counters support")
Signed-off-by: Xiang Yang <xiangyang3@huawei.com>
Link: https://lore.kernel.org/r/20230804022525.1916766-1-xiangyang3@huawei.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>