zfs-test/mmap_seek: fix build on musl
The build on musl needs linux/fs.h for SEEK_DATA and friends,
and sys/sysmacros.h for P2ROUNDUP. Add the needed headers.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Georgy Yakovlev <gyakovlev@gentoo.org>
Closes #12891