xfs: simplify buffer I/O submission

The code in _xfs_buf_ioapply is unnecessarily complicated because it
doesn't take advantage of modern bio features.

Simplify it by making use of bio splitting and chaining, that is build
a single bio for the pages in the buffer using a simple loop, and then
split that bio on the map boundaries for discontiguous multi-FSB buffers
and chain the split bios to the main one so that there is only a single
I/O completion.

This not only simplifies the code to build the buffer, but also removes
the need for the b_io_remaining field as buffer ownership is granted
to the bio on submit of the final bio with no chance for a completion
before that as well as the b_io_error field that is now superfluous
because there always is exactly one completion.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
Acked-by: Dave Chinner <[email protected]>
Signed-off-by: Carlos Maiolino <[email protected]>

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