This ioctl shrinks a given length (aligned to sections) from end of the
main area. Any cursegs and valid blocks will be moved out before
invalidating the range.

This feature can be used for adjusting partition sizes online.

History of the patch:

Sahitya Tummala:
 - Add this ioctl for f2fs_compat_ioctl() as well.
 - Fix debugfs status to reflect the online resize changes.
 - Fix potential race between online resize path and allocate new data
   block path or gc path.

Others:
 - Rename some identifiers.
 - Add some error handling branches.
 - Clear sbi->next_victim_seg[BG_GC/FG_GC] in shrinking range.
 - Implement this interface as ext4's, and change the parameter from shrunk
bytes to new block count of F2FS.
 - During resizing, force to empty sit_journal and forbid adding new
   entries to it, in order to avoid invalid segno in journal after resize.
 - Reduce sbi->user_block_count before resize starts.
 - Commit the updated superblock first, and then update in-memory metadata
   only when the former succeeds.
 - Target block count must align to sections.
 - Write checkpoint before and after committing the new superblock, w/o
CP_FSCK_FLAG respectively, so that the FS can be fixed by fsck even if
resize fails after the new superblock is committed.
 - In free_segment_range(), reduce granularity of gc_mutex.
 - Add protection on curseg migration.
 - Add freeze_bdev() and thaw_bdev() for resize fs.
 - Remove CUR_MAIN_SECS and use MAIN_SECS directly for allocation.
 - Recover super_block and FS metadata when resize fails.
 - No need to clear CP_FSCK_FLAG in update_ckpt_flags().
 - Clean up the sb and fs metadata update functions for resize_fs.

Geert Uytterhoeven:
 - Use div_u64*() for 64-bit divisions

Arnd Bergmann:
 - Not all architectures support get_user() with a 64-bit argument:
    ERROR: "__get_user_bad" [fs/f2fs/f2fs.ko] undefined!
    Use copy_from_user() here, this will always work.
Signed-off-by: Qiuyang Sun <sunqiuyang@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>