xfs: refactor suffix_kstrtoint()

The mount-api doesn't have a "human unit" parse type yet so the options
that have values like "10k" etc. still need to be converted by the fs.

But the value comes to the fs as a string (not a substring_t type) so
there's a need to change the conversion function to take a character
string instead.

When xfs is switched to use the new mount-api match_kstrtoint() will no
longer be used and will be removed.

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>