RDMA/core: Don't infoleak GRH fields

If dst->is_global field is not set, the GRH fields are not cleared
and the following infoleak is reported.

=====================================================
BUG: KMSAN: kernel-infoleak in instrument_copy_to_user include/linux/instrumented.h:121 [inline]
BUG: KMSAN: kernel-infoleak in _copy_to_user+0x1c9/0x270 lib/usercopy.c:33
 instrument_copy_to_user include/linux/instrumented.h:121 [inline]
 _copy_to_user+0x1c9/0x270 lib/usercopy.c:33
 copy_to_user include/linux/uaccess.h:209 [inline]
 ucma_init_qp_attr+0x8c7/0xb10 drivers/infiniband/core/ucma.c:1242
 ucma_write+0x637/0x6c0 drivers/infiniband/core/ucma.c:1732
 vfs_write+0x8ce/0x2030 fs/read_write.c:588
 ksys_write+0x28b/0x510 fs/read_write.c:643
 __do_sys_write fs/read_write.c:655 [inline]
 __se_sys_write fs/read_write.c:652 [inline]
 __ia32_sys_write+0xdb/0x120 fs/read_write.c:652
 do_syscall_32_irqs_on arch/x86/entry/common.c:114 [inline]
 __do_fast_syscall_32+0x96/0xf0 arch/x86/entry/common.c:180
 do_fast_syscall_32+0x34/0x70 arch/x86/entry/common.c:205
 do_SYSENTER_32+0x1b/0x20 arch/x86/entry/common.c:248
 entry_SYSENTER_compat_after_hwframe+0x4d/0x5c

Local variable resp created at:
 ucma_init_qp_attr+0xa4/0xb10 drivers/infiniband/core/ucma.c:1214
 ucma_write+0x637/0x6c0 drivers/infiniband/core/ucma.c:1732

Bytes 40-59 of 144 are uninitialized
Memory access of size 144 starts at ffff888167523b00
Data copied to user address 0000000020000100

CPU: 1 PID: 25910 Comm: syz-executor.1 Not tainted 5.16.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
=====================================================

Fixes: 4ba66093bdc6 ("IB/core: Check for global flag when using ah_attr")
Link: https://lore.kernel.org/r/0e9dd51f93410b7b2f4f5562f52befc878b71afa.1641298868.git.leonro@nvidia.com
Reported-by: syzbot+6d532fa8f9463da290bc@syzkaller.appspotmail.com
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
