f2fs: don't break allocation when crossing contiguous sections

Commit 0638a319 ("f2fs: avoid unused block when dio write in LFS
mode") has fixed unused block issue for dio write in lfs mode.

However, f2fs_map_blocks() may break and return smaller extent when
last allocated block locates in the end of section, even allocator
can allocate contiguous blocks across sections.

Actually, for the case that allocator returns a block address which is
not contiguous w/ current extent, we can record the block address in
iomap->private, in the next round, skip reallocating for the last
allocated block, then we can fix unused block issue, meanwhile, also,
we can allocates contiguous physical blocks as much as possible for dio
write in lfs mode.

Testcase:
- mkfs.f2fs -f /dev/vdb
- mount -o mode=lfs /dev/vdb /mnt/f2fs
- dd if=/dev/zero of=/mnt/f2fs/file bs=1M count=3; sync;
- dd if=/dev/zero of=/mnt/f2fs/dio bs=2M count=1 oflag=direct;
- umount /mnt/f2fs

Before:
f2fs_map_blocks: dev = (253,16), ino = 4, file offset = 0, start blkaddr = 0x0, len = 0x100, flags = 1, seg_type = 8, may_create = 1, multidevice = 0, flag = 5, err = 0
f2fs_map_blocks: dev = (253,16), ino = 4, file offset = 256, start blkaddr = 0x0, len = 0x100, flags = 1, seg_type = 8, may_create = 1, multidevice = 0, flag = 5, err = 0
f2fs_map_blocks: dev = (253,16), ino = 4, file offset = 512, start blkaddr = 0x0, len = 0x100, flags = 1, seg_type = 8, may_create = 1, multidevice = 0, flag = 5, err = 0
f2fs_map_blocks: dev = (253,16), ino = 5, file offset = 0, start blkaddr = 0x4700, len = 0x100, flags = 3, seg_type = 1, may_create = 1, multidevice = 0, flag = 3, err = 0
f2fs_map_blocks: dev = (253,16), ino = 5, file offset = 256, start blkaddr = 0x4800, len = 0x100, flags = 3, seg_type = 1, may_create = 1, multidevice = 0, flag = 3, err = 0

After:
f2fs_map_blocks: dev = (253,16), ino = 4, file offset = 0, start blkaddr = 0x0, len = 0x100, flags = 1, seg_type = 8, may_create = 1, multidevice = 0, flag = 5, err = 0
f2fs_map_blocks: dev = (253,16), ino = 4, file offset = 256, start blkaddr = 0x0, len = 0x100, flags = 1, seg_type = 8, may_create = 1, multidevice = 0, flag = 5, err = 0
f2fs_map_blocks: dev = (253,16), ino = 4, file offset = 512, start blkaddr = 0x0, len = 0x100, flags = 1, seg_type = 8, may_create = 1, multidevice = 0, flag = 5, err = 0
f2fs_map_blocks: dev = (253,16), ino = 5, file offset = 0, start blkaddr = 0x4700, len = 0x200, flags = 3, seg_type = 1, may_create = 1, multidevice = 0, flag = 3, err = 0

Cc: Daejun Park <daejun7.park@samsung.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>