dmu_buf_will_clone: change assertion to fix 32-bit compiler warning

Building module/zfs/dbuf.c for 32-bit targets can result in a warning:

In file included from
/usr/src/sys/contrib/openzfs/include/sys/zfs_context.h:97,

from /usr/src/sys/contrib/openzfs/module/zfs/dbuf.c:32:

/usr/src/sys/contrib/openzfs/module/zfs/dbuf.c: In function
'dmu_buf_will_clone':
/usr/src/sys/contrib/openzfs/lib/libspl/include/assert.h:116:33: error:
cast from pointer to integer of different size
[-Werror=pointer-to-int-cast]

116 |         const uint64_t __left = (uint64_t)(LEFT);
\
    |                                 ^

/usr/src/sys/contrib/openzfs/lib/libspl/include/assert.h:148:25: note:
in expansion of macro 'VERIFY0'

148 | #define ASSERT0         VERIFY0
    |                         ^~~~~~~

/usr/src/sys/contrib/openzfs/module/zfs/dbuf.c:2704:9: note: in
expansion of macro 'ASSERT0'
2704 | ASSERT0(dbuf_find_dirty_eq(db, tx->tx_txg));

      |         ^~~~~~~

This is because dbuf_find_dirty_eq() returns a pointer, which if
pointers are 32-bit results in a warning about the cast to uint64_t.

Instead, use the ASSERT3P() macro, with == and NULL as second and third
arguments, which should work regardless of the target's bitness.

Reviewed-by: Kay Pedersen <mail@mkwg.de>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Dimitry Andric <dimitry@andric.com>
Closes #15224