Change iomap_page_mkwrite() return type to vm_fault_t.

see commit 1c8f4220 ("mm: change return type to vm_fault_t") for
reference.

Link: http://lkml.kernel.org/r/20180827172050.GA18673@jordon-HP-15-Notebook-PCSigned-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Matthew Wilcox <mawilcox@microsoft.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>