PR17190: accept functions given with their decl_line

PR17190: accept functions given with their decl_line

If the user gives a probe for a specific function, e.g.

kernel.function("vfs_read@fs/read_write.c:393")

where the vfs_read() function is declared at lineno 393, we don't
actually want to probe lineno 393 (most likely there's nothing there
anyway). We simply need to probe the function's entry point.