f2fs: clean up w/ fscrypt_is_bounce_page()

[ Upstream commit 0c708e35cf26449ca317fcbfc274704660b6d269 ]

Just cleanup, no logic changes.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>