cifsd: remove useless error handling in ksmbd_vfs_read

dentry->d_inode never happen to be NULL if we hold the dentry.

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>