libzfs: use mount_setattr for selective remount including legacy mounts

When a namespace property is changed via zfs set, libzfs remounts the
filesystem to propagate the new VFS mount flags. The current approach
uses mount(2) with MS_REMOUNT, which reads all namespace properties
from ZFS and applies them together. This has two problems:

1. Linux VFS resets unspecified per-mount flags on remount. If an
   administrator sets a temporary flag (e.g. mount -o remount,noatime),
   a subsequent zfs set on any namespace property clobbers it.

2. Two concurrent zfs set operations on different namespace properties
   can overwrite each other's mount flags.

Additionally, legacy datasets (mountpoint=legacy) were never remounted
on namespace property changes since zfs_is_mountable() returns false
for them.

Add zfs_mount_setattr() which uses mount_setattr(2) to selectively
update only the mount flags that correspond to the changed property.
For legacy datasets, /proc/mounts is iterated to update all
mountpoints. On kernels without mount_setattr (ENOSYS), non-legacy
datasets fall back to a full remount; legacy mounts are skipped to
avoid clobbering temporary flags.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #18257

lib/libzfs/os/freebsd/libzfs_zmount.c

@@ -116,6 +116,17 @@ do_unmount(zfs_handle_t *zhp, const char *mntpt, int flags)
 	return (0);
 }
 
+/*
+ * FreeBSD does not support mount_setattr(2).  Fall back to a full
+ * remount so that the updated namespace property takes effect.
+ */
+int
+zfs_mount_setattr(zfs_handle_t *zhp, uint32_t nspflags)
+{
+	(void) nspflags;
+	return (zfs_mount(zhp, MNTOPT_REMOUNT, 0));
+}
+
 int
 zfs_mount_delegation_check(void)
 {