Fix uninitialized scalar value read regression in dmu_recv_begin()

Fix uninitialized scalar value read regression in dmu_recv_begin()

da19d919a853ad05ef300fe000e6c96c4db84bcf changed this in a way that
permits execution to reach `if (err == 0)` without initializing err.
This could randomly cause the sync task to not execute. We fix that by
initializing err to zero.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reported-by: Coverity (CID-1535377)
Closes #14607