Implement efidp_size

efidp_size will return the size, in bytes, of a EFI device path
structure. This is a convenience wrapper in the same style as the
other linux routines. It's implemented by GetDevicePathSize from EDK2
we already needed for other things.

Sponsored by: Netflix

lib/libefivar/efivar-dp-format.c

@@ -2430,3 +2430,9 @@ efidp_format_device_path(char *buf, size_t len, const_efidp dp, ssize_t max)
 
 	return retval;
 }
+
+size_t
+efidp_size(const_efidp dp)
+{
+	return GetDevicePathSize(__DECONST(EFI_DEVICE_PATH_PROTOCOL *, dp));
+}

lib/libefivar/efivar-dp.h

@@ -62,4 +62,6 @@ ssize_t efidp_format_device_path(char *buf, size_t len, const_efidp dp,
     ssize_t max);
 ssize_t efidp_parse_device_path(char *path, efidp out, size_t max);
 
+size_t efidp_size(const_efidp);
+
 #endif /* _EFIVAR_DP_H_ */