find-esp: do not fail when /boot on btrfs RAID on searching ESP or xbootldr

When /boot or friends is on btrfs RAID, btrfs_get_block_device_at() will
succeed with 0 and provide zero devnum. Then,
- if we are previleged, devname_from_devnum() maps the devnum to
  /run/systemd/inaccessible/blk, and the subsequent verification by blkid
  will fail,
- if we are unprevileged, sd_device_new_from_devnum() will fail.

This makes
- when find_esp() or find_xbootldr() is called without any paths, that
  is, called with the searching mode, then returns -ENOKEY, which should
  be handled gracefully by the caller,
- when they are called with an input path, then they provide the proper
  error message and suggestion.

Fixes RHBZ#2251262 (https://bugzilla.redhat.com/show_bug.cgi?id=2251262).

src/shared/find-esp.c

@@ -396,6 +396,11 @@ static int verify_esp(
         if (relax_checks)
                 goto finish;
 
+        if (devnum_is_zero(devid))
+                return log_full_errno(searching ? LOG_DEBUG : LOG_ERR,
+                                      SYNTHETIC_ERRNO(searching ? EADDRNOTAVAIL : ENODEV),
+                                      "Could not determine backing block device of directory \"%s\" (btrfs RAID?).", p);
+
         /* If we are unprivileged we ask udev for the metadata about the partition. If we are privileged we
          * use blkid instead. Why? Because this code is called from 'bootctl' which is pretty much an
          * emergency recovery tool that should also work when udev isn't up (i.e. from the emergency shell),
@@ -767,6 +772,15 @@ static int verify_xbootldr(
         if (relax_checks)
                 goto finish;
 
+        if (devnum_is_zero(devid))
+                return log_full_errno(searching ? LOG_DEBUG : LOG_ERR,
+                                      SYNTHETIC_ERRNO(searching ? EADDRNOTAVAIL : ENODEV),
+                                      "Could not determine backing block device of directory \"%s\" (btrfs RAID?).%s",
+                                      p,
+                                      searching ? "" :
+                                      "\nHint: set $SYSTEMD_RELAX_XBOOTLDR_CHECKS=yes environment variable "
+                                      "to bypass this and further verifications for the directory.");
+
         if (unprivileged_mode)
                 r = verify_xbootldr_udev(devid, flags, ret_uuid);
         else