f2fs: don't set FI_PREALLOCATED_ALL for partial write

In f2fs_preallocate_blocks(), if it is partial write in 4KB, it's not
necessary to call f2fs_map_blocks() and set FI_PREALLOCATED_ALL flag.

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