Linux 6.6 compat: fix implicit conversion error with debug build

With Linux v6.6.0 and GCC 12, when debug build is configured,
implicit conversion error is raised while converting
'enum <anonymous>' to 'boolean_t'. Use 'B_TRUE' instead of
'true' to fix the issue.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Pavel Snajdr <snajpa@snajpa.net>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #15489