XArray: Turn xa_init_flags into a static inline

A regular xa_init_flags() put all dynamically-initialised XArrays into
the same locking class.  That leads to lockdep believing that taking
one XArray lock while holding another is a deadlock.  It's possible to
work around some of these situations with separate locking classes for
irq/bh/regular XArrays, and SINGLE_DEPTH_NESTING, but that's ugly, and
it doesn't work for all situations (where we have completely unrelated
XArrays).

Signed-off-by: Matthew Wilcox <willy@infradead.org>