xfs: further FSSETXATTR cleanups

Now that we've refactored a lot of the FSSETXATTR parameter checking
code into generic functions, shorten the XFS FS_IOC_[GS]ETFLAGS
implementations by making them wrappers around FS_IOC_FS[GS]ETXATTR.
Finally, kill all the useless support code that prepared the memory
manager to change the S_DAX flag since none of it works anyway.