xfs_scrub: move read verification scheduling to phase6.c

xfs_scrub: move read verification scheduling to phase6.c

Right now there's a weird coupling between read_verify.c and spacemap.c:
Anyone using a read_verify_pool is required to tell the pool how many
threads it's going to use to call read_verify_schedule_io.  This is
because the read_verify_pool accumulates verification requests on a
per-thread basis to try to combine adjacent written regions for media
verification.  However, the verification requests are made from the
phase6.c callback (check_rmap) that is called from the workers created
by scrub_scan_all_spacemaps.

Yeah, that's confusing: implementation details of spacemap.c must be
inferred by phase6.c and passed to read_verify.c.

Let's fix this by moving the per-thread schedule accumulation to
phase6.c before the next patches constrain the number of IO threads
sending verification requests to the kernel.

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