LU-1812 ext4: ec_type has been dropped from ext4_ext_cache

LU-1812 ext4: ec_type has been dropped from ext4_ext_cache

In short, EXT4_EXT_CACHE_EXTENT is indicated when both
ee_len != 0 and ee_start != 0.

see kernel commit b05e6ae58a13b56e3e11882c1fc71948c9b29760

Signed-off-by: chas williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Change-Id: I88bb07c0e09ea13ac17399edb47de8e16f105543
Reviewed-on: http://review.whamcloud.com/4970
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Peng Tao <bergwolf@gmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>