proc: use the same treatment to check proc_lseek as ones for proc_read_iter et.al

In the Linux kernel, the following vulnerability has been resolved:

proc: use the same treatment to check proc_lseek as ones for proc_read_iter et.al

Check pde->proc_ops->proc_lseek directly may cause UAF in rmmod scenario. It's a gap in proc_reg_open() after commit 654b33ada4ab("proc: fix UAF in proc_get_inode()"). Followed by AI Viro's suggestion, fix it in same manner.