xfs_repair: fix 'bno' shadow vars in scan.c

xfs_repair: fix 'bno' shadow vars in scan.c

scan.c has 3 functions which accept 'bno' as an argument, but then use a
local variable of the same name for a different purpose in an inner scope,
which causes sparse warnings.
Rename these inner-scope loop variables 'agbno' to fix this.

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