fs: simplify iget & friends

Merge get_new_inode/get_new_inode_fast into iget5_locked/iget_locked
as those were the only callers.  Remove the internal ifind/ifind_fast
helpers - ifind_fast only had a single caller, and ifind had two
callers wanting it to do different things.  Also clean up the comments
in this area to focus on information important to a developer trying
to use it, instead of overloading them with implementation details.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>