nsfs: use inode_just_drop()

Currently nsfs uses the default inode_generic_drop() fallback which
drops the inode when it's unlinked or when it's unhashed. Since nsfs
never hashes inodes that always amounts to dropping the inode.

But that's just annoying to have to reason through every time we look at
this code. Switch to inode_just_drop() which always drops the inode
explicitly. This also aligns the behavior with pidfs which does the
same.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-2-2e6f823ebdc0@kernel.org
Tested-by: syzbot@syzkaller.appspotmail.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>