ext4: fix copy paste error in ext4_swap_extents()

This bug was found by a static code checker tool for copy paste
problems.

Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Signed-off-by: Vaneet Narang <v.narang@samsung.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>