f2fs: use memcpy_{to,from}_page() where possible

[ Upstream commit b87846bd61c7c09560617da416208a5454530d57 ]

This is simpler, and as a side effect it replaces several uses of
kmap_atomic() with its recommended replacement kmap_local_page().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Stable-dep-of: b1b9896718bc ("fs: f2fs: initialize fsdata in pagecache_write()")
Signed-off-by: Sasha Levin <sashal@kernel.org>