sysdeps: linux: Add BTRFS_SUPER_MAGIC to pathconf

btrfs has a 65535 maximum link count. Include this value in pathconf to
give the real max link count for this filesystem.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

sysdeps/unix/sysv/linux/linux_fsinfo.h

@@ -251,6 +251,7 @@
 #define XFS_SUPER_MAGIC		0x58465342
 
 /* Maximum link counts.  */
+#define BTRFS_LINK_MAX		65535
 #define COH_LINK_MAX		10000
 #define EXT2_LINK_MAX		32000
 #define EXT4_LINK_MAX		65000

sysdeps/unix/sysv/linux/pathconf.c

@@ -186,6 +186,9 @@ __statfs_link_max (int result, const struct statfs *fsbuf, const char *file,
     case LUSTRE_SUPER_MAGIC:
       return LUSTRE_LINK_MAX;
 
+    case BTRFS_SUPER_MAGIC:
+      return BTRFS_LINK_MAX;
+
     default:
       return LINUX_LINK_MAX;
     }