deprecate md bitmap file support

[Christoph Hellwig <hch@lst.de>]

Hi Song,

the md bitmap file support is very problematic in how it bypasses the
file system for file access.  I looked into fixing it in preparation
for making buffer_head optionals but had to give up because it is so
convoluted.  This series includes the cleanups I've started which seem
useful even for the internal bitmap support, then makes the bitmap file
support conditional and adds a deprecation warning.

Diffstat:
 Kconfig     |   10 +
 md-bitmap.c |  339 ++++++++++++++++++++++++++++++------------------------------
 md-bitmap.h |    1 
 md.c        |    9 +
 4 files changed, 191 insertions(+), 168 deletions(-)