ext4: invalidate extent status tree during extent migration

mext_replace_branches() will change inode's extents layout so
we have to drop corresponding cache.

TESTCASE:  301'th xfstest was not yet accepted to official xfstest's branch
and can be found here: https://github.com/dmonakhov/xfstests/commit/7b7efeee30a41109201e2040034e71db9b66ddc0

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>