fscontext: add custom-prefix log helpers

fscontext: add custom-prefix log helpers

Sometimes, errors associated with an fscontext come from the VFS or
otherwise outside of the filesystem driver itself. However, the default
logging of errorfc will always prefix the message with the filesystem
name.

So, add some *fcp() wrappers that allow for custom prefixes to be used
when emitting information to the fscontext log.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/20250806-errorfc-mount-too-revealing-v2-1-534b9b4d45bb@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>