ext4: move_extent_per_page() cleanup

(cherry picked from commit ac48b0a1)

Integrate duplicate lines (acquire/release semaphore and invalidate
extent cache in move_extent_per_page()) into mext_replace_branches(),
to reduce source and object code size.

Signed-off-by: Akira Fujita <a-fujita@rs.jp.nec.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>