btrfs: move extent_buffer::lock_owner to debug section

The lock_owner is used for a rare corruption case and we haven't seen
any reports in years. Move it to the debugging section of eb.  To close
the holes also move log_index so the final layout looks like:

struct extent_buffer {
        u64                        start;                /*     0     8 */
        long unsigned int          len;                  /*     8     8 */
        long unsigned int          bflags;               /*    16     8 */
        struct btrfs_fs_info *     fs_info;              /*    24     8 */
        spinlock_t                 refs_lock;            /*    32     4 */
        atomic_t                   refs;                 /*    36     4 */
        int                        read_mirror;          /*    40     4 */
        s8                         log_index;            /*    44     1 */

        /* XXX 3 bytes hole, try to pack */

        struct callback_head       callback_head __attribute__((__aligned__(8))); /*    48    16 */
        /* --- cacheline 1 boundary (64 bytes) --- */
        struct rw_semaphore        lock;                 /*    64    40 */
        struct page *              pages[16];            /*   104   128 */

        /* size: 232, cachelines: 4, members: 11 */
        /* sum members: 229, holes: 1, sum holes: 3 */
        /* forced alignments: 1, forced holes: 1, sum forced holes: 3 */
        /* last cacheline: 40 bytes */
} __attribute__((__aligned__(8)));

This saves 8 bytes in total and still keeps the lock on a separate cacheline.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>