efi/esrt: Allow ESRT access without CAP_SYS_ADMIN

Access to the files in /sys/firmware/efi/esrt has been restricted to
CAP_SYS_ADMIN since support for ESRT was added, but this seems overly
restrictive given that the files are read-only and just provide
information about UEFI firmware updates.

Remove the CAP_SYS_ADMIN restriction so that a non-root process can read
the files, provided a suitably-privileged process changes the file
ownership first. The files are still read-only and still owned by root
by default.

Signed-off-by: Nicholas Bishop <nicholasbishop@google.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>