squashfs: make squashfs_cache_init() return ERR_PTR(-ENOMEM)

Patch series "squashfs: reduce memory usage and update docs".

This patchset reduces the amount of memory that Squashfs uses when
CONFIG_FILE_DIRECT is configured, and updates various out of date
information in the documentation and Kconfig.

This patch (of 4):

Make squashfs_cache_init() return an ERR_PTR(-ENOMEM) on failure rather
than NULL.

This tidies up some calling code, but, it also allows NULL to be returned
as a valid result when a cache hasn't be allocated.

Link: https://lkml.kernel.org/r/20241229233752.54481-1-phillip@squashfs.org.uk
Link: https://lkml.kernel.org/r/20241229233752.54481-2-phillip@squashfs.org.uk


Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>