LU-11044 osd-ldiskfs: ext4_dir_operations uses iterate_shared

LU-11044 osd-ldiskfs: ext4_dir_operations uses iterate_shared

Linux 4.7 commit ae05327a00fd47c34dfe25294b359a3f3fef96e8 replaces
ext4_dir_operations iterate with iterate_shared. dir_relaxed_shared()
was also added in that commit, so we can use that function to verify
that the ext4_dir_operations is using iterate_shared.

Cray-bug-id: LUS-6008
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I67ff714296cab96408cb74fba62855c0e12cdf43
Reviewed-on: https://review.whamcloud.com/32486
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>