ext4: simple cleanup in fiemap codepath

This commit is simple cleanup of fiemap codepath which has not been
included in previous commit to make the changes clearer. In this commit
we rename cbex variable to newex in ext4_fill_fiemap_extents() because
callback is no longer present

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>