Set mode explicitely when -n command line is specified

This prevents us from doing the wrong thing in such (very unlikely) cases:
$ inotail -c 6 -n 6 file.txt

1 files changed, 1 insertions, 0 deletions

diff --git a/inotail.c b/inotail.c
index cba84c6..fc9fb7e 100644
--- a/inotail.c
+++ b/inotail.c
@@ -320,6 +320,7 @@ int main(int argc, char **argv)
 			forever = 1;
 			break;
 		case 'n':
+			mode = M_LINES;
 			n_lines = strtoul(argv[++opt], NULL, 0);
 			if (n_lines < 0)
 				n_lines = 0;