cifsd: fix reference count decrement of unclaimed file in __ksmbd_lookup_fd

__ksmbd_lookup_fd could decrement the reference count of
unclaimed ksmbd_file to 0 but not release this ksmbd_file.

ksmbd_file cannot be unclaimed except ksmbd_close_inode_fds(),
because ksmbd_file is only removed from the m_fp_list list
after the reference count of ksmbd_file becomes 0. And if the
count is 0, __ksmbd_lookup_fd does not use ksmbd_file found
from idr due to atomic_inc_not_zero.

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>