iomap: Inline __iomap_zero_iter into its caller

To make the merge easier, replicate the inlining of __iomap_zero_iter()
into iomap_zero_iter() that is currently in the nvdimm tree.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>