fs/read_write.c: fix generic_file_llseek() comment

Commit ef3d0fd2 ("vfs: do (nearly) lockless generic_file_llseek")
has removed i_mutex from generic_file_llseek, so update the comment
accordingly.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>