xfs_quota: check for size parsing errors

xfs_quota: check for size parsing errors

Doing something like

# xfs_quota -x -c 'limit -u bhard=1.2g ...

will cause cvtnum to fail and return a value of -1LL (because it
cannot parse the decimal), but the quota caller doesn't check
for this error value, casts it to U64, shifts right, and we end
up with an answer of 16 petabytes rather than erroring out.
Fix this.

Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Reported-by: James Lawrie <james@jdlawrie.co.uk>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>