nilfs2: do not use unexported cpu_to_le32()/le32_to_cpu() in uapi header

commit c32cc30c upstream.

cpu_to_le32/le32_to_cpu is defined in include/linux/byteorder/generic.h,
which is not exported to user-space.

UAPI headers must use the ones prefixed with double-underscore.

Detected by compile-testing exported headers:

  include/linux/nilfs2_ondisk.h: In function `nilfs_checkpoint_set_snapshot':
  include/linux/nilfs2_ondisk.h:536:17: error: implicit declaration of function `cpu_to_le32' [-Werror=implicit-function-declaration]
    cp->cp_flags = cpu_to_le32(le32_to_cpu(cp->cp_flags) |  \
                   ^
  include/linux/nilfs2_ondisk.h:552:1: note: in expansion of macro `NILFS_CHECKPOINT_FNS'
   NILFS_CHECKPOINT_FNS(SNAPSHOT, snapshot)
   ^~~~~~~~~~~~~~~~~~~~
  include/linux/nilfs2_ondisk.h:536:29: error: implicit declaration of function `le32_to_cpu' [-Werror=implicit-function-declaration]
    cp->cp_flags = cpu_to_le32(le32_to_cpu(cp->cp_flags) |  \
                               ^
  include/linux/nilfs2_ondisk.h:552:1: note: in expansion of macro `NILFS_CHECKPOINT_FNS'
   NILFS_CHECKPOINT_FNS(SNAPSHOT, snapshot)
   ^~~~~~~~~~~~~~~~~~~~
  include/linux/nilfs2_ondisk.h: In function `nilfs_segment_usage_set_clean':
  include/linux/nilfs2_ondisk.h:622:19: error: implicit declaration of function `cpu_to_le64' [-Werror=implicit-function-declaration]
    su->su_lastmod = cpu_to_le64(0);
                     ^~~~~~~~~~~

Link: http://lkml.kernel.org/r/20190605053006.14332-1-yamada.masahiro@socionext.com
Fixes: e63e88bc

 ("nilfs2: move ioctl interface and disk layout to uapi separately")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Joe Perches <joe@perches.com>
Cc: <stable@vger.kernel.org>	[4.9+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>