fs: fs_open can corrupt fs_open_t object given via zfp parameter

[de-nordic]

Describe the bug
There is no check, within fs_open, whether the provided zfp already points to used object.
This means that when fs_open is used twice on the same object it will overwrite contents of
zfp before passing execution to file system driver.
This may lead to two possible outcomes, depending on whether zfp is used for opening (1) file with the same path or (2) file with different path.
According to fs_api tests (1) should end with an error, but this is actually left to file system driver and is not certain.
In both cases the file system driver will be called which may result in resource leak; e.g. LittleFS will allocate slab memory overwriting original pointer within provided fs_file_t object before further deciding if open is successful or not; this means that at this point, if there has been already memory allocated by previous fs_open, if such occurred.
Depending on whether error occurred or not, and on stage, the pointers may be pointing to new resources, that might have been already released and is invalid, or NULL

To Reproduce
Using LittleFS mounted partition.
Call fs_open, on different or the same file, twice and observe fs_file_t->filep.

Expected behavior
Not breaking the original structure and returning with error instead.

Impact
Possible data corruption, resource leaks, double assignment of the same file system driver structures to different file streams.

Workaround
Always fs_close fs_file_t before reusing structure. Avoid double-open as even on error the original fs_file_t object will be damaged.

Environment (please complete the following information):
-- zephyrproject-rtos/zephyr: c13603e

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