xfs/291: fix spurious ENOSPC errors

xfs/291: fix spurious ENOSPC errors

xfs/291 tries to fill the filesystem almost full, so if the log size
changes with mkfs defaults then it's free space calculations are not
longer valid and so it throws lots of ENOSPC errors during a run.
This is not fatal for this test, but it does increase the runtime of
it and fill the 291.full file with unnecessary errors.

The number of frag files it creates is also too many for a 512 byte
inode filesystem (by about 900) so reduce the number of inodes
initially created so the test works ofr 512 byte inodes. With 512
byte inodes, the free space histogram looks like this after the frag
phase:

   from      to extents  blocks    pct
      1       1   10730   10730 100.00

And for 256 byte inodes:

   from      to extents  blocks    pct
      1       1   12388   12388 100.00

So these changes do not affect the intended operation of the test.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>