UPSTREAM: f2fs: Use sysfs_emit_at() to simplify code

This file already uses sysfs_emit(). So be consistent and also use
sysfs_emit_at().

This slightly simplifies the code and makes it more readable.

Reviewed-by: Chao Yu <chao@kernel.org>
Change-Id: Ic0fff15c2a2a8d7b4eb8ca16bf89889881be3208
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit f7a678bb)