iomap: Use kmap_local_page instead of kmap_atomic

kmap_atomic() has the side-effect of disabling pagefaults and
preemption.  kmap_local_page() does not do this and is preferred.

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