Fix compat with linux > 6.1 headers
    
    In file included from /usr/include/linux/btrfs.h:29,
                     from ../linux-user/syscall.c:163:
    /usr/include/linux/fs.h:50:8: error: redefinition of 'struct file_clone_range'
       50 | struct file_clone_range {
          |        ^~~~~~~~~~~~~~~~
    ../linux-user/syscall.c:129:8: note: originally defined here
      129 | struct file_clone_range {
          |        ^~~~~~~~~~~~~~~~
    
    was caused by btrfs.h gaining an include of linux/fs.h,
    which clashed with QEMU's workaround for glibc compat
    with linux/mount.h.  GLibc huas fixed their sys/mount.h
    so we can drop the QEMU workaround, thus fixing the
    btrfs.h problem.
    
    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>