xfs: strengthen validation of extent size hints

While playing around with realtime extent sizes and extent size hints, I
noticed that it was very possible for userspace to trip the inode
verifiers if they tried to set an extent size hint that wasn't aligned
to the rt extent size and then create realtime files.  This series
tightens the existing checks and refactors the ioctls to use the libxfs
validation functions like the verifiers, mkfs, and repair use.

For v2, we also detect invalid extent size hints on existing filesystems
and mitigate the problem by (a) not propagating the invalid hints to new
realtime files and (b) removing invalid hints when set on directories.