statx: Include a mask for stx_attributes in struct statx

Include a mask in struct stat to indicate which bits of stx_attributes the
filesystem actually supports.

This would also be useful if we add another system call that allows you to
do a 'bulk attribute set' and pass in a statx struct with the masks
appropriately set to say what you want to set.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/uapi/linux/stat.h

@@ -114,7 +114,7 @@ struct statx {
 	__u64	stx_ino;	/* Inode number */
 	__u64	stx_size;	/* File size */
 	__u64	stx_blocks;	/* Number of 512-byte blocks allocated */
-	__u64	__spare1[1];
+	__u64	stx_attributes_mask; /* Mask to show what's supported in stx_attributes */
 	/* 0x40 */
 	struct statx_timestamp	stx_atime;	/* Last access time */
 	struct statx_timestamp	stx_btime;	/* File creation time */
@@ -155,7 +155,7 @@ struct statx {
 #define STATX__RESERVED		0x80000000U	/* Reserved for future struct statx expansion */
 
 /*
- * Attributes to be found in stx_attributes
+ * Attributes to be found in stx_attributes and masked in stx_attributes_mask.
  *
  * These give information about the features or the state of a file that might
  * be of use to ordinary userspace programs such as GUIs or ls rather than