xfs_io: fix pwrite/pread length truncation on 32-bit systems

xfs_io: fix pwrite/pread length truncation on 32-bit systems

The pwrite and pread commands in xfs_io accept an operation length that
can be any quantity that fits in a long long int; and loops to handle
the cases where the operation length is larger than the IO buffer.

Weirdly, the do_ functions contain code to shorten the operation to the
IO buffer size but the @count parameter is size_t, which means that for
a large argument on a 32-bit system, we rip off the upper bits of the
length, turning your 8GB write into a 0 byte write, which does nothing.

This was found by running generic/175 and observing that the 8G test
file it creates has zero length after the operation:

wrote 0/8589934592 bytes at offset 0
0.000000 bytes, 0 ops; 0.0001 sec (0.000000 bytes/sec and 0.0000 ops/sec)

Fix this by pushing long long count all the way through the call stack.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>