subsys: fs/nvs: remove explicit padding from structures

Now that the flash writes are padded up to the write block size, there
is no need to have explicit padding fields in the _nvs_sector_hdr and
_nvs_data_slt structure. This allow to save space when the write block
size equals to 1 or 2

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>