netfs, fscache: export fscache_put_volume() and add fscache_try_get_volume()

[ Upstream commit 85b08b31

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Export fscache_put_volume() and add fscache_try_get_volume()
helper function to allow cachefiles to get/put fscache_volume
via linux/fscache-cache.h.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Link: https://lore.kernel.org/r/20240628062930.2467993-2-libaokun@huaweicloud.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
Stable-dep-of: 522018a0 ("cachefiles: fix slab-use-after-free in fscache_withdraw_volume()")
Stable-dep-of: 5d8f8057

 ("cachefiles: fix slab-use-after-free in cachefiles_withdraw_cookie()")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>