linux-user: Protect btrfs ioctl target definitions

Target definitions of btrfs ioctls in 'syscall_defs.h' use
the value BTRFS_IOCTL_MAGIC that is defined header 'btrfs.h'.
This header is not available in kernel versions before 3.9.
For that reason, these target ioctl definitions should be
enwrapped in an #ifdef directive to check whether the 'btrfs.h'
header is available as to not cause build errors on older
Linux systems.

Signed-off-by: Filip Bozuta <Filip.Bozuta@syrmia.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200905163802.2666-1-Filip.Bozuta@syrmia.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

linux-user/syscall_defs.h

@@ -1006,6 +1006,7 @@ struct target_rtc_pll_info {
 #define TARGET_FS_IOC32_SETVERSION TARGET_IOW('v', 2, int)
 
 /* btrfs ioctls */
+#ifdef CONFIG_BTRFS
 #define TARGET_BTRFS_IOC_SNAP_CREATE            TARGET_IOWU(BTRFS_IOCTL_MAGIC, 1)
 #define TARGET_BTRFS_IOC_SCAN_DEV               TARGET_IOWU(BTRFS_IOCTL_MAGIC, 4)
 #define TARGET_BTRFS_IOC_FORGET_DEV             TARGET_IOWU(BTRFS_IOCTL_MAGIC, 5)
@@ -1041,6 +1042,7 @@ struct target_rtc_pll_info {
 #define TARGET_BTRFS_IOC_GET_SUBVOL_INFO        TARGET_IORU(BTRFS_IOCTL_MAGIC, 60)
 #define TARGET_BTRFS_IOC_GET_SUBVOL_ROOTREF     TARGET_IOWRU(BTRFS_IOCTL_MAGIC, 61)
 #define TARGET_BTRFS_IOC_INO_LOOKUP_USER        TARGET_IOWRU(BTRFS_IOCTL_MAGIC, 62)
+#endif
 
 /* usb ioctls */
 #define TARGET_USBDEVFS_CONTROL TARGET_IOWRU('U', 0)