vfs: support FS_XFLAG_COWEXTSIZE and get/set of CoW extent size hint

Introduce XFLAGs for the new XFS CoW extent size hint, and actually
plumb the CoW extent size hint into the fsxattr structure.

Signed-off-by: Darrick J. Wong <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>

include/uapi/linux/fs.h

···
 	__u32		fsx_extsize;	/* extsize field value (get/set)*/
 	__u32		fsx_nextents;	/* nextents field value (get)	*/
 	__u32		fsx_projid;	/* project identifier (get/set) */
-	unsigned char	fsx_pad[12];
+	__u32		fsx_cowextsize;	/* CoW extsize field value (get/set)*/
+	unsigned char	fsx_pad[8];
 };
 
 /*
···
 #define FS_XFLAG_NODEFRAG	0x00002000	/* do not defragment */
 #define FS_XFLAG_FILESTREAM	0x00004000	/* use filestream allocator */
 #define FS_XFLAG_DAX		0x00008000	/* use DAX for IO */
+#define FS_XFLAG_COWEXTSIZE	0x00010000	/* CoW extent size allocator hint */
 #define FS_XFLAG_HASATTR	0x80000000	/* no DIFLAG for this	*/
 
 /* the read-only stuff doesn't really belong here, but any other place is