iomap: remove set_memor_ro() on zero page

Stephen reported a boot failure on ppc power8 system where
set_memor_ro() on the new zero page failed [0]. Christophe Leroy
further clarifies we can't use this on on linear memory on ppc, and
so instead of special casing this just for PowerPC [2] remove the
call as suggested by Darrick.

[0] https://lore.kernel.org/all/20240826175931.1989f99e@canb.auug.org.au/T/#u
[1] https://lore.kernel.org/all/b0fe75b4-c1bb-47f7-a7c3-2534b31c1780@csgroup.eu/
[2] https://lore.kernel.org/all/ZszrJkFOpiy5rCma@bombadil.infradead.org/



Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Suggested-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20240826212632.2098685-1-mcgrof@kernel.org


Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>