f2fs: replace the debugfs_root with f2fs_debugfs_root

This minor change for the naming conventions of debugfs_root
to avoid any possible conflicts to the other filesystem.

Signed-off-by: Younger Liu <younger.liucn@gmail.com>
Cc: Younger Liu <younger.liucn@gmail.com>
Cc: Jaegeuk Kim <jaegeuk.kim@samsung.com>
[Jaegeuk Kim: change the patch name]
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>