ext4: correctly calculate number of blocks for fiemap

ext4: correctly calculate number of blocks for fiemap

commit aca92ff6f57c000d1b4523e383c8bd6b8269b8b1 upstream.

ext4_fiemap() rounds the length of the requested range down to
blocksize, which is is not the true number of blocks that cover the
requested region.  This problem is especially impressive if the user
requests only the first byte of a file: not a single extent will be
reported.

We fix this by calculating the last block of the region and then
subtract to find the number of blocks in the extents.

Signed-off-by: Leonard Michlmayr <leonard.michlmayr@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>