IB/core: const'ify inbuf in struct ib_udata

Userspace input buffer is not modified by kernel, so it can be 'const'.

This is also a prerequisite to remove the implicit cast
from INIT_UDATA().

Link: http://marc.info/?i=cover.1386798254.git.ydroneaud@opteya.com

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Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>