LU-2555 fsfilt: fix race in ldiskfs_ext_new_extent_cb

LU-2555 fsfilt: 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()).

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: Ie28f7dfc393c8feb8336f6b391cebb0adc814283
Reviewed-on: http://review.whamcloud.com/4977
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>