xfs: Code cleanup and removal of some typedef usage

In preparation for combined pquota/gquota support, for the sake
of readability, do some code cleanup surrounding the affected
code.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>