Btrfs: hold the tree mod lock in __tree_mod_log_rewind

commit f1ca7e98a67da618d8595866e0860308525154da upstream.

We need to hold the tree mod log lock in __tree_mod_log_rewind since we walk
forward in the tree mod entries, otherwise we'll end up with random entries and
trip the BUG_ON() at the front of __tree_mod_log_rewind.  This fixes the panics
people were seeing when running

find /whatever -type f -exec btrfs fi defrag {} \;

Thansk,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>