libspl: move ptob() from zfs_context.h

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17861

lib/libspl/include/os/freebsd/sys/param.h

@@ -58,6 +58,8 @@
 extern size_t spl_pagesize(void);
 #define	PAGESIZE	(spl_pagesize())
 
+#define	ptob(x)		((x) * PAGESIZE)
+
 #ifndef HAVE_EXECVPE
 extern int execvpe(const char *name, char * const argv[], char * const envp[]);
 #endif

lib/libspl/include/os/linux/sys/param.h

@@ -65,4 +65,6 @@
 extern size_t spl_pagesize(void);
 #define	PAGESIZE	(spl_pagesize())
 
+#define	ptob(x)		((x) * PAGESIZE)
+
 #endif