ext4: remove redundant o_start statement

ext4: remove redundant o_start statement

The if will goto out of the loop, and until the end of the
function execution, o_start will not be used again.

The clang_analyzer complains as follows:

fs/ext4/move_extent.c:635:5 warning:

Value stored to 'o_start' is never read

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: luo penghao <luo.penghao@zte.com.cn>
Link: https://lore.kernel.org/r/20211208075157.404535-1-luo.penghao@zte.com.cn
Signed-off-by: Theodore Ts'o <tytso@mit.edu>