fs/aio: Stop allocating aio rings from HIGHMEM

There is no need to allocate aio rings from HIGHMEM because of very
little memory needed here.

Therefore, use GFP_USER flag in find_or_create_page() and get rid of
kmap*() mappings.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ira Weiny <ira.weiny@intel.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Message-Id: <20230609145937.17610-1-fmdefrancesco@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>