fs_parser: move fsparam_string_empty() helper into header

Since both ext4 and overlayfs define the same macro to specify string
parameters that may allow empty values, define it in an header file so
that this helper can be shared.

Signed-off-by: Luis Henriques (SUSE) <luis.henriques@linux.dev>
Link: https://lore.kernel.org/r/20240312104757.27333-1-luis.henriques@linux.dev


Signed-off-by: Christian Brauner <brauner@kernel.org>