pNFS/NFSv4: Remove redundant initialization of 'rd_size'

Variable 'rd_size' is being initialized however
this value is never read as 'rd_size' is assigned
a new value in for statement. Remove the redundant
assignment.

Clean up clang warning:

fs/nfs/pnfs.c:2681:6: warning: Value stored to 'rd_size' during its
initialization is never read [clang-analyzer-deadcode.DeadStores]

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>