mkfs.xfs: print std info if agcount makes agsize out of bounds

mkfs.xfs: print std info if agcount makes agsize out of bounds

When specifying a too-small agcount with stripe geometry,
mkfs.xfs can fail with a somewhat unexpected message:

$ mkfs.xfs -f -d file,name=fsfile,size=9764864000b,agcount=31,su=512k,sw=20
Allocation group size (314995613) is not a multiple of the stripe unit (128)

This strikes me as especially odd because normally, mkfs.xfs
tries to fix up the agsize to be a stripe multiple.  The only way
we get to the above error message is if ag _size_ is out of bounds;
exiting with an error about alignment rather than about size
seems odd.

Maybe below is too clever, but if by the time we've decided that
agsize is out of bounds after rounding it both up and down,
as necessary, to get to a stripe-width multiple, calling
validate_ag_geometry() will give us the same standard message as
if we had specified no stripe geometry:

$ mkfs/mkfs.xfs -f -d file,name=fsfile,size=9764864000b,agcount=31,su=512k,sw=20
agsize (314995613b) too big, maximum is 268435455 blocks
Usage: mkfs.xfs
...

$ mkfs/mkfs.xfs -f -d file,name=fsfile,size=9764864000b,agcount=31
agsize (314995613b) too big, maximum is 268435455 blocks
Usage: mkfs.xfs
...

Also, tidy up error message to explicitly state "blocks" not "b"

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>