squashfs: don't call kmalloc in decompressors

The decompressors may be called while in an atomic section.  So move the
kmalloc() out of this path, and into the "page actor" init function.

This fixes a regression introduced by commit
f268eedd ("squashfs: extend "page actor" to handle missing pages")

Link: https://lkml.kernel.org/r/20220822215430.15933-1-phillip@squashfs.org.uk
Fixes: f268eedd

 ("squashfs: extend "page actor" to handle missing pages")
Reported-by: Chris Murphy <lists@colorremedies.com>
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>