iomap: pass writeback errors to the mapping

Modern-day mapping_set_error has the ability to squash the usual
negative error code into something appropriate for long-term storage in
a struct address_space -- ENOSPC becomes AS_ENOSPC, and everything else
becomes EIO.  iomap squashes /everything/ to EIO, just as XFS did before
that, but this doesn't make sense.

Fix this by making it so that we can pass ENOSPC to userspace when
writeback fails due to space problems.

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