common/rc: factor out _ext4_disable_extent_zeroout() helper

common/rc: factor out _ext4_disable_extent_zeroout() helper

1) This pattern is repeated in several seek_data/hole tests
   (e.g. generic/285, generic/436, generic/445 generic/448)
   and generic/009.  A common _ext4_disable_extent_zeroout()
   helper could be added and applied by generic/009 and
   _require_seek_data_hole().

2) On some old kernels(e.g. v3.1-v3.6), when vfs recognizes
   SEEK_DATA/HOLE flag && ext4 has no extent zeroout tunable
   in sysfs, these cases may trigger "sysfs entry not found"
   issue.  We can add check if extent_max_zeroout_kb exists
   on ext4 filesystem.
   The extent_max_zeroout_kb is introduced by:
   '67a5da564f97 ("ext4: make the zero-out chunk size tunable")'

3) Declare several vars as local in _require_seek_data_hole().

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>