build direct-io.c conditionally

[Christoph Hellwig]

Hi all,

this series makes the build of direct-io.c conditional as only
about a dozen file systems actually use it.

Diffstat:
 Kconfig          |    4 ++++
 Makefile         |    3 ++-
 affs/Kconfig     |    1 +
 direct-io.c      |   24 ------------------------
 exfat/Kconfig    |    1 +
 ext2/Kconfig     |    1 +
 fat/Kconfig      |    1 +
 hfs/Kconfig      |    1 +
 hfsplus/Kconfig  |    1 +
 internal.h       |    4 +---
 jfs/Kconfig      |    1 +
 nilfs2/Kconfig   |    1 +
 ntfs3/Kconfig    |    1 +
 ocfs2/Kconfig    |    1 +
 reiserfs/Kconfig |    1 +
 super.c          |   24 ++++++++++++++++++++++++
 udf/Kconfig      |    1 +
 17 files changed, 43 insertions(+), 28 deletions(-)