xfs_quota: refactor code to generate id from name

xfs_quota: refactor code to generate id from name

There's boilerplate for setting limits and warnings, where we have
a case statement for each of the 3 quota types, and from there call
3 different functions to configure each of the 3 types, each of which
calls its own version of id to string function...

Refactor this so that the main function can call a generic id to string
conversion routine, and then call a common action.  This save a lot of
LOC.

I was looking at allowing xfs to bump out individual grace periods like
setquota can do, and this refactoring allows us to add new actions like
that without copying all the boilerplate again.

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