Fix gcc self-comparison warning

As of gcc 6.1.1 20160621 (Red Hat 6.1.1-3) a self-comparison is
detected by gcc in metaslab_alloc(). Resolve the warning by passing
a physical size of 0 to BP_SET_BIRTH() as it done by other callers.

module/zfs/metaslab.c: In function ‘metaslab_alloc’:
module/zfs/metaslab.c:2575:184: error: self-comparison always evaluates
    to true [-Werror=tautological-compare]

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Issue #4907