VM_HUGEPAGE support for XFS

[Matthew Wilcox]

Today I decided to implement VM_HUGEPAGE support for XFS.  It turned out
to be a rather simpler implementation than I was expecting because I
could reuse the readahead implementation.

Feel free to try it for yourself:
http://git.infradead.org/users/willy/pagecache.git

The patches up to "fs: Do not update nr_thps for mappings which support... "
are in linux-next for 5.10.  I hope to get the rest into 5.11.