ioctl_fat.2: Correctly reference volume ID instead of volume label

The description of FAT_IOCTL_GET_VOLUME_ID references volume name,
when it really should be volume ID since the volume label is
different in the FAT filesystem. This patch renames the incorrect
volume name references to volume id.

This man page supplies an example display_fat_volume_id.c program
that can be used to read the volume ID (serial number) from a
filesystem.  Here is an additional test showing that the volume ID
and volume name are two different entities:

$ dd if=/dev/zero of=fat_volume bs=1M count=1
$ mkfs.fat -v -n MASNEYB fat_volume
[snip]
Volume ID is da8cecf2, volume label MASNEYB    .
$ sudo mount -o loop fat_volume /mnt
$ ./display_fat_volume_id /mnt/
Volume ID da8c-ecf2

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/ioctl_fat.2

@@ -88,9 +88,9 @@ It is reset by an archiving system.
 The zero value
 .B ATTR_NONE
 can be used to indicate that no attribute bit is set.
-.SS Reading the volume label
-FAT filesystems are identified by a volume label.
-The volume label can be read with
+.SS Reading the volume ID
+FAT filesystems are identified by a volume ID.
+The volume ID can be read with
 .BR FAT_IOCTL_GET_VOLUME_ID .
 .PP
 The
@@ -106,7 +106,7 @@ flag.
 The
 .I id
 argument is a pointer to the field that will be filled with the volume ID.
-Typically the volume label is displayed to the user as a group of two
+Typically the volume ID is displayed to the user as a group of two
 16-bit fields:
 .PP
 .in +4n
@@ -347,10 +347,10 @@ main(int argc, char *argv[])
 }
 .fi
 .in
-.SS Reading the volume label
+.SS Reading the volume ID
 The following program demonstrates the use of
 .BR ioctl (2)
-to display the volume label of a FAT filesystem.
+to display the volume ID of a FAT filesystem.
 .PP
 The following output was recorded when applying the program for
 directory