RDMA/uverbs: Fix typo of sizeof argument

RDMA/uverbs: Fix typo of sizeof argument

Since size of 'hdr' pointer and '*hdr' structure is equal on 64-bit
machines issue probably didn't cause any wrong behavior. But anyway,
fixing of typo is required.

Fixes: da0f60df7bd5 ("RDMA/uverbs: Prohibit write() calls with too small buffers")
Co-developed-by: Ivanov Mikhail <ivanov.mikhail1@huawei-partners.com>
Signed-off-by: Ivanov Mikhail <ivanov.mikhail1@huawei-partners.com>
Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Link: https://lore.kernel.org/r/20230905103258.1738246-1-konstantin.meskhidze@huawei.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>