vfs: output mount_too_revealing() errors to fscontext

vfs: output mount_too_revealing() errors to fscontext

It makes little sense for fsmount() to output the warning message when
mount_too_revealing() is violated to kmsg. Instead, the warning should
be output (with a "VFS" prefix) to the fscontext log. In addition,
include the same log message for mount_too_revealing() when doing a
regular mount for consistency.

With the newest fsopen()-based mount(8) from util-linux, the error
messages now look like

  # mount -t proc proc /tmp
  mount: /tmp: fsmount() failed: VFS: Mount too revealing.
         dmesg(1) may have more information after failed mount system call.

which could finally result in mount_too_revealing() errors being easier
for users to detect and understand.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/20250806-errorfc-mount-too-revealing-v2-2-534b9b4d45bb@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>