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

tests/zfs-tests/cmd/mmap_seek/mmap_seek.c

@@ -29,7 +29,11 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/mman.h>
+#include <sys/sysmacros.h>
 #include <errno.h>
+#ifdef __linux__
+#include <linux/fs.h>
+#endif
 
 static void
 seek_data(int fd, off_t offset, off_t expected)