fuse: add fuse_ prefix to several functions

Add fuse_ prefix to request_send*() and get_root_inode() as some of
those functions will be exported for CUSE.  With or without CUSE
export, having the function names scoped is a good idea for
debuggability.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>