Return boolean_t in inline functions of lib/libspl/include/sys/uio.h

The inline functions zfs_dio_offset_aligned(), zfs_dio_size_aligned()
and zfs_dio_aligned() are declared as boolean_t but return the bool
type.

This fixes the build of FreeBSD.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Martin Matuska <mm@FreeBSD.org>
Closes #16613