fs: use namespace_{lock,unlock} in dissolve_on_fput()

In commit b73ec10 ("fs: add fastpath for dissolve_on_fput()"),
the namespace_{lock,unlock} has been replaced with scoped_guard
using the namespace_sem. This however now also skips processing of
'unmounted' list in namespace_unlock(), and mount is not (immediately)
cleaned up.

For example, this causes LTP move_mount02 fail:
    ...
    move_mount02.c:80: TPASS: invalid-from-fd: move_mount() failed as expected: EBADF (9)
    move_mount02.c:80: TPASS: invalid-from-path: move_mount() failed as expected: ENOENT (2)
    move_mount02.c:80: TPASS: invalid-to-fd: move_mount() failed as expected: EBADF (9)
    move_mount02.c:80: TPASS: invalid-to-path: move_mount() failed as expected: ENOENT (2)
    move_mount02.c:80: TPASS: invalid-flags: move_mount() failed as expected: EINVAL (22)
    tst_test.c:1833: TINFO: === Testing on ext3 ===
    tst_test.c:1170: TINFO: Formatting /dev/loop0 with ext3 opts='' extra opts=''
    mke2fs 1.47.2 (1-Jan-2025)
    /dev/loop0 is apparently in use by the system; will not make a filesystem here!
    tst_test.c:1170: TBROK: mkfs.ext3 failed with exit code 1

The test makes number of move_mount() calls but these are all designed to fail
with specific errno. Even after test, 'losetup -d' can't detach loop device.

Define a new guard for dissolve_on_fput, that will use namespace_{lock,unlock}.

Fixes: b73ec10 ("fs: add fastpath for dissolve_on_fput()")
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Link: https://lore.kernel.org/cad2f042b886bf0ced3d8e3aff120ec5e0125d61.1744297468.git.jstancek@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: jstancek

fs/namespace.c

@@ -1830,6 +1830,8 @@ static inline void namespace_lock(void)
 	down_write(&namespace_sem);
 }
 
+DEFINE_GUARD(namespace_lock, struct rw_semaphore *, namespace_lock(), namespace_unlock())
+
 enum umount_tree_flags {
 	UMOUNT_SYNC = 1,
 	UMOUNT_PROPAGATE = 2,
@@ -2383,7 +2385,7 @@ void dissolve_on_fput(struct vfsmount *mnt)
 			return;
 	}
 
-	scoped_guard(rwsem_write, &namespace_sem) {
+	scoped_guard(namespace_lock, &namespace_sem) {
 		ns = m->mnt_ns;
 		if (!must_dissolve(ns))
 			return;