fs: RCU-ify filesystems list

The drivers list was protected by an rwlock; every mount, every open
of /proc/filesystems and the legacy sysfs(2) syscall walked a
hand-rolled singly-linked list under it. /proc/filesystems is
especially hot because libselinux causes programs as mundane as
mkdir, ls and sed to open and read it on every invocation.

Convert the list to an RCU-protected hlist and switch the writer side
to a plain spinlock. Writers keep their existing non-sleeping
section while readers walk under rcu_read_lock() with no lock traffic:

  - register_filesystem()/unregister_filesystem() take
    file_systems_lock, publish via hlist_{add_tail,del_init}_rcu()
    and invalidate the cached /proc/filesystems string.
    unregister_filesystem() keeps its synchronize_rcu() after
    dropping the lock so in-flight readers are drained before the
    module (and its embedded file_system_type) can go away.

  - __get_fs_type(), list_bdev_fs_names() and the
    fs_index()/fs_name()/fs_maxindex() helpers walk the list under
    rcu_read_lock(). fs_name() continues to drop the read-side
    lock after try_module_get() and accesses ->name outside the RCU
    section; the module reference pins the embedded file_system_type
    across the boundary.

struct file_system_type::next becomes struct hlist_node list; no
in-tree caller references the old ->next field outside
fs/filesystems.c.

Link: https://patch.msgid.link/20260425220844.1763933-3-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

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