xfs/123: fix remote symlink block size calculation

xfs/123: fix remote symlink block size calculation

In this test we try to create a remote symlink block by creating a
symlink target buffer large enough to exceed the size of an inode.
Unfortunately we don't use the correct block size or symlink header
size, which on a 1k block filesystem causes there to be two remote
blocks.  This causes crc verification errors in xfs_db (because it's
too dumb to load both blocks as one like the kernel does) which we
don't care about because we're about to corrupt the block anyway.

So, fix the block size calculation so that we end up with one block.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>