xfs: track the number of blocks in each buftarg

Add a bt_nr_sectors to track the number of sector in each buftarg, and
replace the check that hard codes sb_dblock in xfs_buf_map_verify with
this new value so that it is correct for non-ddev buftargs.  The
RT buftarg only has a superblock in the first block, so it is unlikely
to trigger this, or are we likely to ever have enough blocks in the
in-memory buftargs, but we might as well get the check right.

Fixes: 10616b80 ("xfs: fix _xfs_buf_find oops on blocks beyond the filesystem end")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>