zcommon: pre-iterate over sysfs instead of statting every feature

If sufficient memory (<2K, realistically) is available, libzfs_init()
can be significantly shorted by iterating over the correct sysfs
directory before registrations, we can turn 168 stats into 15/18
syscalls (3 opens (6 if built in), 3 fstats, 6 getdentses, and 3
closes), a tenfoldish reduction; this is probably a bit faster, too.

The list is always optional, and registration functions (and one-off
users) can simply pass NULL, which will fall back to the previous
mechanism

Also, don't allocate in zfs_mod_supported_impl, and use use access()
instead of stat(), since existence is really what we care about

Also, fix pre-prop-checking compat in fallback for built-in ZFS

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12089

include/sys/zfs_sysfs.h

@@ -25,6 +25,10 @@
 #ifndef	_SYS_ZFS_SYSFS_H
 #define	_SYS_ZFS_SYSFS_H extern __attribute__((visibility("default")))
 
+struct zfs_mod_supported_features;
+struct zfs_mod_supported_features *zfs_mod_list_supported(const char *scope);
+void zfs_mod_list_supported_free(struct zfs_mod_supported_features *);
+
 #ifdef _KERNEL
 
 void zfs_sysfs_init(void);
@@ -35,7 +39,8 @@ void zfs_sysfs_fini(void);
 #define	zfs_sysfs_init()
 #define	zfs_sysfs_fini()
 
-_SYS_ZFS_SYSFS_H boolean_t zfs_mod_supported(const char *, const char *);
+_SYS_ZFS_SYSFS_H boolean_t zfs_mod_supported(const char *, const char *,
+    const struct zfs_mod_supported_features *);
 #endif
 
 #define	ZFS_SYSFS_POOL_PROPERTIES	"properties.pool"