Change the order of SHMEM initialization and reservation locking
to make code cleaner and to prepare for transitioning of the common
GEM SHMEM code to use the GEM's reservation lock instead of the
shmem.page_lock.

There is no need to lock reservation during allocation of the SHMEM pages
because the lock is needed only to avoid racing with the async host-side
allocation. Hence we can safely move the SHMEM initialization out of the
reservation lock.
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20220630200726.1884320-7-dmitry.osipenko@collabora.comSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>