RDMA/mlx5: Check that supplied blue flame index doesn't overflow

User's supplied index is checked again total number of system pages, but
this number already includes num_static_sys_pages, so addition of that
value to supplied index causes to below error while trying to access
sys_pages[].

BUG: KASAN: slab-out-of-bounds in bfregn_to_uar_index+0x34f/0x400
Read of size 4 at addr ffff880065561904 by task syz-executor446/314

CPU: 0 PID: 314 Comm: syz-executor446 Not tainted 4.18.0-rc1+ #256
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
Call Trace:
 dump_stack+0xef/0x17e
 print_address_description+0x83/0x3b0
 kasan_report+0x18d/0x4d0
 bfregn_to_uar_index+0x34f/0x400
 create_user_qp+0x272/0x227d
 create_qp_common+0x32eb/0x43e0
 mlx5_ib_create_qp+0x379/0x1ca0
 create_qp.isra.5+0xc94/0x22d0
 ib_uverbs_create_qp+0x21b/0x2a0
 ib_uverbs_write+0xc2c/0x1010
 vfs_write+0x1b0/0x550
 ksys_write+0xc6/0x1a0
 do_syscall_64+0xa7/0x590
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x433679
Code: fd ff 48 81 c4 80 00 00 00 e9 f1 fe ff ff 0f 1f 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 3b 91 fd ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff2b3d8e48 EFLAGS: 00000217 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00000000004002f8 RCX: 0000000000433679
RDX: 0000000000000040 RSI: 0000000020000240 RDI: 0000000000000003
RBP: 00000000006d4018 R08: 00000000004002f8 R09: 00000000004002f8
R10: 00000000004002f8 R11: 0000000000000217 R12: 0000000000000000
R13: 000000000040cb00 R14: 000000000040cb90 R15: 0000000000000006

Allocated by task 314:
 kasan_kmalloc+0xa0/0xd0
 __kmalloc+0x1a9/0x510
 mlx5_ib_alloc_ucontext+0x966/0x2620
 ib_uverbs_get_context+0x23f/0xa60
 ib_uverbs_write+0xc2c/0x1010
 __vfs_write+0x10d/0x720
 vfs_write+0x1b0/0x550
 ksys_write+0xc6/0x1a0
 do_syscall_64+0xa7/0x590
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 1:
 __kasan_slab_free+0x12e/0x180
 kfree+0x159/0x630
 kvfree+0x37/0x50
 single_release+0x8e/0xf0
 __fput+0x2d8/0x900
 task_work_run+0x102/0x1f0
 exit_to_usermode_loop+0x159/0x1c0
 do_syscall_64+0x408/0x590
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff880065561100
 which belongs to the cache kmalloc-4096 of size 4096
The buggy address is located 2052 bytes inside of
 4096-byte region [ffff880065561100, ffff880065562100)
The buggy address belongs to the page:
page:ffffea0001955800 count:1 mapcount:0 mapping:ffff88006c402480 index:0x0 compound_mapcount: 0
flags: 0x4000000000008100(slab|head)
raw: 4000000000008100 ffffea0001a7c000 0000000200000002 ffff88006c402480
raw: 0000000000000000 0000000080070007 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff880065561800: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff880065561880: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff880065561900: 04 fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                   ^
 ffff880065561980: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff880065561a00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

Cc: <stable@vger.kernel.org> # 4.15
Fixes: 1ee47ab3 ("IB/mlx5: Enable QP creation with a given blue flame index")
Reported-by: Noa Osherovich <noaos@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>