fs: fat: consistent error handling for flush_dir()

Provide function description for flush_dir().
Move all error messages for flush_dir() from the callers to the function.
Move mapping of errors to -EIO to the function.
Always check return value of flush_dir() (Coverity CID 316362).

In fat_unlink() return -EIO if flush_dirty_fat_buffer() fails.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>