fs: make unlazy_walk() error handling consistent

Most callers check for non-zero return, and assume it's -ECHILD (which
it always will be). One caller uses the actual error return. Clean this
up and make it fully consistent, by having unlazy_walk() return a bool
instead. Rename it to try_to_unlazy() and return true on success, and
failure on error. That's easier to read.

No functional changes in this patch.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>