IB/hfi1: use size_t for passing array length

gcc-7 produces a mysterious warning about the size argument being potentially out
of range:

drivers/infiniband/hw/hfi1/verbs.c: In function 'init_cntr_names':
drivers/infiniband/hw/hfi1/verbs.c:1644:2: error: 'memcpy': specified size between 18446744071562067968 and 18446744073709551615 exceeds maximum object size 9223372036854775807 [-Werror=stringop-overflow=]

This seems to refer to a the case where an 64-bit size_t gets truncated
into a negative 'int' and subsequently turned into a high 64-bit number
again.

The fix is clearly to use size_t here, which matches the type that gets
used for this value elsewhere.

Fixes: b7481944 ("IB/hfi1: Show statistics counters under IB stats interface")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>