subsys/fs: Fix fs_* operations crashing when file closed

Attempt to perform fs_read, fs_write, fs_seek, fs_tell, fs_truncate
and fs_sync on file that has been closed, prior to attempt, would cause
NULL pointer dereference.
With this fix, such operations would instead return -EBADF.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>