This commit moves all operations on single files into ZVFS and makes the
POSIX subsystem call into ZVFS to perform them. It was necessary to define
a `struct zvfs_stat` to avoid a dependency cycle. Functions used
internally for file i/o operations are publicised since they won't require
any changes between various subsystems. This allows ZVFS to actually
fulfill its purpose of facilitating cooperation of different file APIs.
Signed-off-by: Jakub Klimczak <jklimczak@internships.antmicro.com>
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