xfs_quota: warn if specified non-zero quota will be round down to zero

xfs_quota: warn if specified non-zero quota will be round down to zero

User specified quota limit is internally converted to "basic blocks"
unit (512 bytes in size). Quota value will be silently converted
to zero when user enters any value lower than 512 bytes.

Warn in such case:

: # ./xfs_quota -x -c "limit -u bsoft=431 12000" /home
: xfs_quota: Warning: `431' in quota blocks is 0 (unlimited).

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>