Since deduplication potentially has to read in all the pages in both
files in order to compare the contents, cap the deduplication request
length at MAX_RW_COUNT/2 (roughly 1GB) so that we have /some/ upper bound
on the request length and can't just lock up the kernel forever.  Found
by running generic/304 after commit 1ddae54555b62 ("common/rc: add
missing 'local' keywords").

Reported-by: matorola@gmail.com
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>