iomap: add a merge boundary flag

File systems might have boundaries over which merges aren't possible.
In fact these are very common, although most of the time some kind of
header at the beginning of this region (e.g. XFS alloation groups, ext4
block groups) automatically create a merge barrier.  But if that is
not present, say for a device purely used for data we need to manually
communicate that to iomap.

Add a IOMAP_F_BOUNDARY flag to never merge I/O into a previous mapping.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>