Btrfs: don't put removals from push_node_left into tree mod log twice

Independant of the check (push_items < src_items) tree_mod_log_eb_copy did
log the removal of the old data entries from the source buffer. Therefore,
we must not call tree_mod_log_eb_move if the check evaluates to true, as
that would log the removal twice, finally resulting in (rewinded) buffers
with wrong values for header_nritems.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>