pstore: fix one type of return value in pstore

the return type of function _read_ in pstore is size_t,
but in the callback function of _read_, the logic doesn't
consider it too much, which means if negative value (assuming
error here) is returned, it will be converted to positive because
of type casting. ssize_t is enough for this function.

Signed-off-by: Chen Gong <gong.chen@linux.intel.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>

drivers/acpi/apei/erst.c

@@ -929,7 +929,7 @@ static int erst_check_table(struct acpi_table_erst *erst_tab)
 	return 0;
 }
 
-static size_t erst_reader(u64 *id, enum pstore_type_id *type,
+static ssize_t erst_reader(u64 *id, enum pstore_type_id *type,
 		       struct timespec *time);
 static u64 erst_writer(enum pstore_type_id type, size_t size);
 
@@ -957,7 +957,7 @@ struct cper_pstore_record {
 	char data[];
 } __packed;
 
-static size_t erst_reader(u64 *id, enum pstore_type_id *type,
+static ssize_t erst_reader(u64 *id, enum pstore_type_id *type,
 		       struct timespec *time)
 {
 	int rc;