dlmfs: clean up dlmfs_file_{read,write}() a bit

The damn file is constant-sized - 64 bytes.  IOW,
	* i_size_read() is pointless
	* so's dynamic allocation
	* so's the 'size' argument of user_dlm_read_lvb()
	* ... and so's open-coding simple_read_from_buffer(), while we are at it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>