sysfs has always had a bug that would allow a read-only file to be opened 
for writing. It has also returned 0 on write when there was no store method
defined for the file. 

This addresses both via sysfs_open_file(). It checks the flags the file was
opened with and compares them with the mode of the inode. If the mode does
not support the flags passed, -EPERM is returned.

If the sysfs_ops for the object does not have the correct method for the 
flags, -EACCESS is returned. 

Since all checks happen on open(), the corresponding checks in the read()
and write() methods have been removed.