lib/os: fdtable: add locking to posix api

Added locking to posix read(), write(), close()
for additional protection.

In read() missing lock would create uneven calls to locking
mechanism in sockets.c after k_condvar_wait().
That results in socket lock not ever being unlocked

Signed-off-by: Daniel Nejezchleb <dnejezchleb@hwg.cz>