capabilities: fix undefined behavior in bit shift for CAP_TO_MASK

Shifting signed 32-bit value by 31 bits is undefined, so changing
significant bit to unsigned. The UBSAN warning calltrace like below:

UBSAN: shift-out-of-bounds in security/commoncap.c:1252:2
left shift of 1 by 31 places cannot be represented in type 'int'
Call Trace:
 <TASK>
 dump_stack_lvl+0x7d/0xa5
 dump_stack+0x15/0x1b
 ubsan_epilogue+0xe/0x4e
 __ubsan_handle_shift_out_of_bounds+0x1e7/0x20c
 cap_task_prctl+0x561/0x6f0
 security_task_prctl+0x5a/0xb0
 __x64_sys_prctl+0x61/0x8f0
 do_syscall_64+0x58/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
 </TASK>

Fixes: e338d26 ("Add 64-bit capability support to the kernel")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Acked-by: Andrew G. Morgan <morgan@kernel.org>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

include/uapi/linux/capability.h

@@ -426,7 +426,7 @@ struct vfs_ns_cap_data {
  */
 
 #define CAP_TO_INDEX(x)     ((x) >> 5)        /* 1 << 5 == bits in __u32 */
-#define CAP_TO_MASK(x)      (1 << ((x) & 31)) /* mask for indexed __u32 */
+#define CAP_TO_MASK(x)      (1U << ((x) & 31)) /* mask for indexed __u32 */
 
 
 #endif /* _UAPI_LINUX_CAPABILITY_H */