f2fs: do not preallocate blocks which has wrong buffer

Sheng Yong reports needless preallocation if write(small_buffer, large_size)
is called.

In that case, f2fs preallocates large_size, but vfs returns early due to
small_buffer size. Let's detect it before preallocation phase in f2fs.

Change-Id: I932f9d442c3bad6576ab67ddca47d500edf905fe
Reported-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Git-Repo: https://android.googlesource.com/kernel/common/


Git-Commit: b6453fcb
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>