fs: copy BTRFS_IOC_[SG]ET_FSLABEL to vfs

This retains 256 chars as the maximum size through the interface, which
is the btrfs limit and AFAIK exceeds any other filesystem's maximum
label size.

This just copies the ioctl for now and leaves it in place for btrfs
for the time being.  A later patch will allow btrfs to use the new
common ioctl definition, but it may be sent after this is merged.

(Note, Reviewed-by's were originally given for the combined vfs+btrfs
patch, some license taken here.)

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: David Sterba <dsterba@suse.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

Documentation/ioctl/ioctl-number.txt

@@ -296,7 +296,8 @@ Code  Seq#(hex)	Include File		Comments
 0x90	00	drivers/cdrom/sbpcd.h
 0x92	00-0F	drivers/usb/mon/mon_bin.c
 0x93	60-7F	linux/auto_fs.h
-0x94	all	fs/btrfs/ioctl.h
+0x94	all	fs/btrfs/ioctl.h	Btrfs filesystem
+		and linux/fs.h		some lifted to vfs/generic
 0x97	00-7F	fs/ceph/ioctl.h		Ceph file system
 0x99	00-0F				537-Addinboard driver
 					<mailto:buk@buks.ipn.de>