Settings FILE subsystem is not working with SD card with FAT filesystem

[Mierunski]

Describe the bug
Settings FILE subsystem is not working with SD card with FAT filesystem, providing file directory with /SD:/ prefix results in fail to load or create settings file.

I'm testing on nRF9160DK with external SD Card module.

When settings file subsystem is trying to create directory for file, it checks directories if they exist one by one.
Because mount point is first directory in path, fs_stat returns -ENOENT after that fs_mkdir is called which returns -EEXIST
https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/settings/src/settings_file.c#L524

For workaround, i tested substitiuting expression in https://github.com/zephyrproject-rtos/zephyr/blob/main/subsys/settings/src/settings_file.c#L548 to err && err != -EEXIST
After that code is working, but it does not seem like a final solution

To Reproduce
Take a board with SD Card, enable Filesystem backend for setting

CONFIG_SETTINGS=y
CONFIG_SETTINGS_FILE=y
CONFIG_SETTINGS_FILE_PATH="/SD:/setts.ini"
CONFIG_FILE_SYSTEM=y
CONFIG_FAT_FILESYSTEM_ELM=y

Expected behavior
Settings file backend works correctly

Impact
Settings file backend is useless without additional zephyr patch kept separately

Logs and console output

*** Booting nRF Connect SDK v3.5.99-ncs1 ***
[00:00:00.577,209] <inf> sd: Maximum SD clock is under 25MHz, using clock of 8000000Hz

Listing dir /SD: ...
[DIR ] System Volume Information
[FILE] setts.ini (size = 486)
[FILE] global_data.json (size = 124)
[DIR ] log
[00:00:00.590,087] <inf> APP: Application started
path = /SD:, err = -2
[00:00:00.590,209] <err> fs: failed to create directory (-17)
settings_subsys_init failed (err -17)

Environment (please complete the following information):

• OS: Windows
• Toolchain NCS v2.6.0
• Version used v3.5.99-ncs1

[aescolar]

This issue seems to relate to Nordic's Connect SDK.
If the issue is present in Zephyr please provide the information requested in the bug template so the it can be reproduced in upstream Zephyr without NCS.
Otherwise please file the issue downstream instead.

[aescolar]

CC @de-nordic @mniestroj