f2fs: rework write preallocations

f2fs_write_begin() assumes that all blocks were preallocated by
default unless FI_NO_PREALLOC is explicitly set.  This invites data
corruption, as there are cases in which not all blocks are preallocated.
Commit 47501f87

 ("f2fs: preallocate DIO blocks when forcing
buffered_io") fixed one case, but there are others remaining.

Fix up this logic by replacing this flag with FI_PREALLOCATED_ALL, which
only gets set if all blocks for the current write were preallocated.

Also clean up f2fs_preallocate_blocks(), move it to file.c, and make it
handle some of the logic that was previously in write_iter() directly.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>