iomap: move the zeroing case out of iomap_read_page_sync

mainline inclusion
from mainline-v5.5-rc1
commit d3b40439
category: bugfix
bugzilla: 43551
CVE: NA

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That keeps the function a little easier to understand, and easier to
modify for pending enhancements.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

conflicts:
fs/iomap.c

Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>