util/grub-editenv: Add probe call for external envblk

util/grub-editenv: Add probe call for external envblk

This patch adds the probe_fs_envblk() function to identify the root
filesystem and invoke fs_envblk_init() with the probed filesystem type
and device. This checks if the feature is available and initializes the
handle, fs_envblk, to access the external environment block. It avoids
configurations with diskfilter or cryptodisk where filesystem blocks may
be remapped or encrypted.

The probe is only invoked when grub-editenv is working on the default
environment file path. This restriction ensures that probing and
possible raw device access are not triggered for arbitrary user supplied
paths, but only for the standard grubenv file. In that case the code
checks if the filename equals DEFAULT_ENVBLK_PATH and then calls
probe_fs_envblk with fs_envblk_spec. The result is stored in the global
fs_envblk handle. At this stage the external environment block is only
detected and recorded, and the behavior of grub-editenv is unchanged.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>
Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>