src/t_stripealign.c: Fix complier error

src/t_stripealign.c: Fix complier error

The FIEMAP_EXTENT_SHARED flag was added into kernel since commit
8c0414cd524e, so undefined FIEMAP_EXTENT_SHARED resulted in complier
error on old distros(e.g. RHEL6), as below:
----------------------------------------------------------------------
t_stripealign.c:99: error: 'FIEMAP_EXTENT_SHARED' undeclared (first use in this function)
----------------------------------------------------------------------

We fix it by defining the flag manually.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>