Auto merge of #2941 - SteveLauC:statx-constants-on-gnu-linux, r=JohnTitor

add missing STATX_ATTR_* constants on gnu/linux

Corresponds to [this](https://github.com/torvalds/linux/blob/master/include/uapi/linux/stat.h#L189~L191):

```c
#define STATX_ATTR_MOUNT_ROOT		0x00002000 /* Root of a mount */
#define STATX_ATTR_VERITY		0x00100000 /* [I] Verity protected file */
#define STATX_ATTR_DAX			0x00200000 /* File is currently in DAX state */
```

Author: bors

libc-test/build.rs

@@ -3313,6 +3313,9 @@ fn test_linux(target: &str) {
             | "IFLA_ALT_IFNAME"
             | "IFLA_PERM_ADDRESS"
             | "IFLA_PROTO_DOWN_REASON"
+            | "STATX_ATTR_MOUNT_ROOT"
+            | "STATX_ATTR_VERITY"
+            | "STATX_ATTR_DAX"
             if sparc64 => true,
             // Added in Linux 5.13
             "PTRACE_GET_RSEQ_CONFIGURATION" if sparc64 => true,

libc-test/semver/linux-gnu.txt

@@ -424,6 +424,9 @@ STATX_ATTR_COMPRESSED
 STATX_ATTR_ENCRYPTED
 STATX_ATTR_IMMUTABLE
 STATX_ATTR_NODUMP
+STATX_ATTR_MOUNT_ROOT
+STATX_ATTR_VERITY
+STATX_ATTR_DAX
 STATX_BASIC_STATS
 STATX_BLOCKS
 STATX_BTIME

src/unix/linux_like/linux/gnu/mod.rs

@@ -1026,6 +1026,9 @@ pub const STATX_ATTR_APPEND: ::c_int = 0x0020;
 pub const STATX_ATTR_NODUMP: ::c_int = 0x0040;
 pub const STATX_ATTR_ENCRYPTED: ::c_int = 0x0800;
 pub const STATX_ATTR_AUTOMOUNT: ::c_int = 0x1000;
+pub const STATX_ATTR_MOUNT_ROOT: ::c_int = 0x2000;
+pub const STATX_ATTR_VERITY: ::c_int = 0x00100000;
+pub const STATX_ATTR_DAX: ::c_int = 0x00200000;
 
 pub const SOMAXCONN: ::c_int = 4096;