squashfs: don't allocate read_page cache if SQUASHFS_FILE_DIRECT configured

squashfs: don't allocate read_page cache if SQUASHFS_FILE_DIRECT configured

If Squashfs has been configured to directly read datablocks into the page
cache (SQUASHFS_FILE_DIRECT), then the read_page cache is unnecessary.

This improvement is due to the following two commits, which added the
ability to read datablocks into the page cache when pages were missing,
enabling the fallback which used an intermediate buffer to be removed.

commit f268eedddf359 ("squashfs: extend "page actor" to handle missing pages")
commit 1bb1a07afad97 ("squashfs: don't use intermediate buffer if pages missing")

This reduces the amount of memory used when mounting a filesystem by
block_size * maximum number of threads.

Link: https://lkml.kernel.org/r/20241229233752.54481-3-phillip@squashfs.org.uk
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>