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>

fs/ext4/super.c

@@ -1723,10 +1723,6 @@ static const struct constant_table ext4_param_dax[] = {
 	{}
 };
 
-/* String parameter that allows empty argument */
-#define fsparam_string_empty(NAME, OPT) \
-	__fsparam(fs_param_is_string, NAME, OPT, fs_param_can_be_empty, NULL)
-
 /*
  * Mount option specification
  * We don't use fsparam_flag_no because of the way we set the

fs/overlayfs/params.c

@@ -139,10 +139,6 @@ static int ovl_verity_mode_def(void)
 	return OVL_VERITY_OFF;
 }
 
-#define fsparam_string_empty(NAME, OPT) \
-	__fsparam(fs_param_is_string, NAME, OPT, fs_param_can_be_empty, NULL)
-
-
 const struct fs_parameter_spec ovl_parameter_spec[] = {
 	fsparam_string_empty("lowerdir",    Opt_lowerdir),
 	fsparam_string("lowerdir+",         Opt_lowerdir_add),

include/linux/fs_parser.h

@@ -132,4 +132,8 @@ static inline bool fs_validate_description(const char *name,
 #define fsparam_path(NAME, OPT)	__fsparam(fs_param_is_path, NAME, OPT, 0, NULL)
 #define fsparam_fd(NAME, OPT)	__fsparam(fs_param_is_fd, NAME, OPT, 0, NULL)
 
+/* String parameter that allows empty argument */
+#define fsparam_string_empty(NAME, OPT) \
+	__fsparam(fs_param_is_string, NAME, OPT, fs_param_can_be_empty, NULL)
+
 #endif /* _LINUX_FS_PARSER_H */