iomap: don't bother unsharing delalloc extents

iomap: don't bother unsharing delalloc extents

If unshare encounters a delalloc reservation in the srcmap, that means
that the file range isn't shared because delalloc reservations cannot be
reflinked.  Therefore, don't try to unshare them.

Signed-off-by: Darrick J. Wong <[email protected]>
Link: https://lore.kernel.org/r/20241002150040.GB21853@frogsfrogsfrogs
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Brian Foster <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>