xfs: don't map ranges that span EOF for direct IO

Al Viro tracked down the problem that has caused generic/263 to fail
on XFS since the test was introduced. If is caused by
xfs_get_blocks() mapping a single extent that spans EOF without
marking it as buffer-new() so that the direct IO code does not zero
the tail of the block at the new EOF. This is a long standing bug
that has been around for many, many years.

Because xfs_get_blocks() starts the map before EOF, it can't set
buffer_new(), because that causes he direct IO code to also zero
unaligned sectors at the head of the IO. This would overwrite valid
data with zeros, and hence we cannot validly return a single extent
that spans EOF to direct IO.

Fix this by detecting a mapping that spans EOF and truncate it down
to EOF. This results in the the direct IO code doing the right thing
for unaligned data blocks before EOF, and then returning to get
another mapping for the region beyond EOF which XFS treats correctly
by setting buffer_new() on it. This makes direct Io behave correctly
w.r.t. tail block zeroing beyond EOF, and fsx is happy about that.

Again, thanks to Al Viro for finding what I couldn't.

[ dchinner: Fix for __divdi3 build error:

Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
]

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Tested-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>