xattr dataset prop: change defaults to sa

It's the main recommendation to set xattr=sa
even in man pages, so let's set it by default.

xattr=sa don't use feature flag, so in the worst
case we'll have non-readable xattrs by other
non-openzfs platforms.

Non-overridden default `xattr` prop of existing pools
will automatically use `sa` after this commit too.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #15147

man/man7/zfsprops.7

@@ -1971,14 +1971,13 @@ enabled for virus scanning to occur.
 The default value is
 .Sy off .
 This property is not used by OpenZFS.
-.It Sy xattr Ns = Ns Sy on Ns | Ns Sy off Ns | Ns Sy sa
+.It Sy xattr Ns = Ns Sy on Ns | Ns Sy off Ns | Ns Sy dir Ns | Ns Sy sa
 Controls whether extended attributes are enabled for this file system.
 Two styles of extended attributes are supported: either directory-based
 or system-attribute-based.
 .Pp
-The default value of
-.Sy on
-enables directory-based extended attributes.
+Directory-based extended attributes can be enabled by setting the value to
+.Sy dir .
 This style of extended attribute imposes no practical limit
 on either the size or number of attributes which can be set on a file.
 Although under Linux the
@@ -1991,7 +1990,10 @@ This is the most compatible
 style of extended attribute and is supported by all ZFS implementations.
 .Pp
 System-attribute-based xattrs can be enabled by setting the value to
-.Sy sa .
+.Sy sa
+(default and equal to
+.Sy on
+) .
 The key advantage of this type of xattr is improved performance.
 Storing extended attributes as system attributes
 significantly decreases the amount of disk I/O required.