page cache: use xa_lock

Remove the address_space ->tree_lock and use the xa_lock newly added to
the radix_tree_root.  Rename the address_space ->page_tree to ->i_pages,
since we don't really care that it's a tree.

[willy@infradead.org: fix nds32, fs/dax.c]
  Link: http://lkml.kernel.org/r/20180406145415.GB20605@bombadil.infradead.orgLink: http://lkml.kernel.org/r/20180313132639.17387-9-willy@infradead.org
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Cc: Darrick J. Wong <darrick.wong@oracle.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pagemap.h

@@ -144,7 +144,7 @@ void release_pages(struct page **pages, int nr);
  * 3. check the page is still in pagecache (if no, goto 1)
  *
  * Remove-side that cares about stability of _refcount (eg. reclaim) has the
- * following (with tree_lock held for write):
+ * following (with the i_pages lock held):
  * A. atomically check refcount is correct and set it to 0 (atomic_cmpxchg)
  * B. remove page from pagecache
  * C. free the page
@@ -157,7 +157,7 @@ void release_pages(struct page **pages, int nr);
  *
  * It is possible that between 1 and 2, the page is removed then the exact same
  * page is inserted into the same position in pagecache. That's OK: the
- * old find_get_page using tree_lock could equally have run before or after
+ * old find_get_page using a lock could equally have run before or after
  * such a re-insertion, depending on order that locks are granted.
  *
  * Lookups racing against pagecache insertion isn't a big problem: either 1