Squashfs: don't allocate fragment caches more than fragments

Sometimes the actual number of fragments in image is between
0 and SQUASHFS_CACHED_FRAGMENTS, which cause additional
fragment caches to be allocated.

Sets the number of fragment caches to the minimum of fragments
and SQUASHFS_CACHED_FRAGMENTS.

Link: https://lkml.kernel.org/r/20241210090842.160853-1-pangliyuan1@huawei.com


Signed-off-by: pangliyuan <pangliyuan1@huawei.com>
Reviewed-by: Phillip Lougher <phillip@squashfs.org.uk>
Cc: <wangfangpeng1@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>