nvlist: Add nvlist_snprintf() and zfs_dbgmsg_nvlist()

Add nvlist_snprintf() to print a nvlist to a buffer.  This is basically
the snprintf() version of dump_nvlist().  Along with that, add a
zfs_dbgmsg_nvlist() to print out an nvlist to dbgmsg.  This will aid in
debugging.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #17215

module/Kbuild.in

@@ -413,6 +413,7 @@ ZFS_OBJS := \
 	zfeature.o \
 	zfs_byteswap.o \
 	zfs_chksum.o \
+	zfs_debug_common.o \
 	zfs_fm.o \
 	zfs_fuid.o \
 	zfs_impl.o \