xfs: Check for extent overflow when moving extent from cow to data fork

xfs: Check for extent overflow when moving extent from cow to data fork

This test verifies that XFS does not cause inode fork's extent count to
overflow when writing to/funshare-ing a shared extent.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>