fs: shave 8 bytes off of struct inode

Here is a link to Linus' reply to Jan's concern about making
i_blkbibts byte addressable:
https://marc.info/?l=linux-fsdevel&m=152882624707975&w=2

Here is a link to an lkp.org report about potential performance
improvement in some workload, which could(?) be related to packing
i_blkbits closer to i_bytes/i_lock:
https://marc.info/?l=linux-fsdevel&m=153077048108198&w=2

Changes since v1:
- Add links to relevant discussions
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -275,6 +275,7 @@ struct writeback_control;
 
 /*
  * Write life time hint values.
+ * Stored in struct inode as u8.
  */
 enum rw_hint {
 	WRITE_LIFE_NOT_SET	= 0,
@@ -609,8 +610,8 @@ struct inode {
 	struct timespec64	i_ctime;
 	spinlock_t		i_lock;	/* i_blocks, i_bytes, maybe i_size */
 	unsigned short          i_bytes;
-	unsigned int		i_blkbits;
-	enum rw_hint		i_write_hint;
+	u8			i_blkbits;
+	u8			i_write_hint;
 	blkcnt_t		i_blocks;
 
 #ifdef __NEED_I_SIZE_ORDERED