iomap: convert iomap_unshare_iter to use large folios

Convert iomap_unshare_iter to create large folios if possible, since the
write and zeroing paths already do that.  I think this got missed in the
conversion of the write paths that landed in 6.6-rc1.

Cc: ritesh.list@gmail.com, willy@infradead.org
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>