fs: rename struct xattr_ctx to kernel_xattr_ctx

fs: rename struct xattr_ctx to kernel_xattr_ctx

Rename the struct xattr_ctx to increase distinction with the about to be
added user API struct xattr_args.

No functional change.

Suggested-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Link: https://lore.kernel.org/r/20240426162042.191916-2-cgoettsche@seltendoof.de
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>