autofs: replace manual symlink buffer allocation in autofs_dir_symlink

autofs: replace manual symlink buffer allocation in autofs_dir_symlink

The symlink name was previously duplicated using an explicit kmalloc()
followed by strcpy(), which is deprecated [1]. Replace this open-coded
string duplication with kstrdup(), which allocates and copies the
symlink name with a single helper function.

Remove the local variable 'size' and set 'i_size' directly using
strlen(cp), which is equivalent to the previous value of 'size'.

This simplifies the code, uses common string-handling helpers, and
removes the deprecated use of strcpy().

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strcpy
Acked-by: Ian Kent <raven@themaw.net>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20260318001219.2354-3-thorsten.blum@linux.dev
Signed-off-by: Christian Brauner <brauner@kernel.org>