bpf: more trivial fdget() conversions

All failure exits prior to fdget() leave the scope, all matching fdput()
are immediately followed by leaving the scope.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>