VFS_QUOTACTL: Remove needless casts of arg

The argument is a void * so there's no need to cast it to caddr_t.

Update documentation to match function decleration.

Reviewed by: freqlabs
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27093

(cherry picked from commit 52e63ec2f1ddf76c9411b6833bc5d021457b5005)