LU-4853 ldiskfs: fix race in ldiskfs_ext_new_extent_cb

LU-4853 ldiskfs: fix race in ldiskfs_ext_new_extent_cb

In ldiskfs_ext_calc_credits_for_insert(), we should use the 'depth'
stored in the 'path' instead from inode, because the extent tree
could have been changed when ldiskfs_ext_calc_credits_for_insert()
is called (by ldiskfs_ext_new_extent_cb()).

It was fixed in LU-2555, but the fix is missed in
sles11sp2/ext4-misc.patch

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I9ca349ba52060f10fc980721317ba47e10572473
Reviewed-on: http://review.whamcloud.com/9868
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>