common/xfs: Add a helper to export inode core size

common/xfs: Add a helper to export inode core size

Some xfs test cases need the number of bytes reserved for only the inode
record, excluding the immediate fork areas. Now the value is hard-coded
and it is not a good chioce. Add a helper in common/xfs to export the
inode core size.

Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>