xfs: fix len comparison in xfs_extent_busy_trim

The length is now passed by reference, so the assertion has to be updated
to match the other changes, as pointed out by this W=1 warning:

fs/xfs/xfs_extent_busy.c: In function 'xfs_extent_busy_trim':
fs/xfs/xfs_extent_busy.c:356:13: error: ordered comparison of pointer with integer zero [-Werror=extra]

Fixes: ebf55872 ("xfs: improve handling of busy extents in the low-level allocator")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>