LU-3373 osd-ldiskfs: export ext4_truncate

LU-3373 osd-ldiskfs: export ext4_truncate

The latest kernel removes inode_operations::truncate member, while
SLES11 kernel still keep the member but does not fill that member
for regular file.

This patch exports symbol for ext4_truncate, and calls it directly
in osd_punch() if the regular file inode operation does not fill its
truncate function.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I42477ca3f1a56e9c0870a641431936298f6d71b5
Reviewed-on: http://review.whamcloud.com/9174
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>