f2fs: add ioctl to get IO priority hint

This patch adds an ioctl to give a per-file priority hint to attach
REQ_PRIO.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

include/uapi/linux/f2fs.h

@@ -44,6 +44,7 @@
 #define F2FS_IOC_COMPRESS_FILE		_IO(F2FS_IOCTL_MAGIC, 24)
 #define F2FS_IOC_START_ATOMIC_REPLACE	_IO(F2FS_IOCTL_MAGIC, 25)
 #define F2FS_IOC_GET_DEV_ALIAS_FILE	_IOR(F2FS_IOCTL_MAGIC, 26, __u32)
+#define F2FS_IOC_IO_PRIO		_IOW(F2FS_IOCTL_MAGIC, 27, __u32)
 
 /*
  * should be same as XFS_IOC_GOINGDOWN.
@@ -63,6 +64,12 @@
 #define F2FS_TRIM_FILE_ZEROOUT		0x2	/* zero out */
 #define F2FS_TRIM_FILE_MASK		0x3
 
+/* for F2FS_IOC_IO_PRIO */
+enum {
+	F2FS_IOPRIO_WRITE = 1,	/* high write priority */
+	F2FS_IOPRIO_MAX,
+};
+
 struct f2fs_gc_range {
 	__u32 sync;
 	__u64 start;