[patch] small sendfile fixes

[Robert Love]

Three fixes for sendfile, mostly related to sending large files from pseudo
filesystems:

- Fix sendfile for offsets > 300G. This can happen with pseudo filesystems.
  This happens because the overflow check is using inode->i_sb->s_maxbytes
  and not the superblock of the backing device's s_maxbytes. For a regular file
  these are interchangible but for a special file these are different and you
  want the latter.

- Don't compare against the max of the out_inode's superblock. Doesn't make
  sense.

- For pseudo and other filesystems with s_maxbytes set to ~0ULL, max ends up
  holding a negative number as it is signed. Check for and correct that.

Git patch is against 2.6.30. I'd like Al to take a look and make sure
my logic is right before applying.

        Robert

From: Mandeep Singh Baines <msb@xxxxxxxxxx>
Signed-off-by: Robert Love <rlove@xxxxxxxxxx>
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