less than
blocksize blocks (GFS2 for example).  Since we are always mapping at least
blocksize sections anyway, just make sure len is at least as big as a
blocksize so we don't trip up any filesystems.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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