f2fs: support STATX_DIOALIGN

Add support for STATX_DIOALIGN to f2fs, so that direct I/O alignment
restrictions are exposed to userspace in a generic way.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>
Link: https://lore.kernel.org/r/20220827065851.135710-8-ebiggers@kernel.org