f2fs: simplify f2fs_force_buffered_io()

f2fs only allows direct I/O that is aligned to the filesystem block
size.  Given that fact, simplify f2fs_force_buffered_io() by removing
the redundant call to block_unaligned_IO().

This makes it easier to reuse this code for STATX_DIOALIGN.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>
Link: https://lore.kernel.org/r/20220827065851.135710-7-ebiggers@kernel.org