f2fs: add a fast path in finish_preallocate_blocks()

This patch uses i_sem to protect access/update on f2fs_inode_info.flag
in finish_preallocate_blocks(), it avoids grabbing inode_lock() in
each open().

Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>