nfsd: call op_release, even when op_func returns an error

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

nfsd: call op_release, even when op_func returns an error

For ops with "trivial" replies, nfsd4_encode_operation will shortcut

most of the encoding work and skip to just marshalling up the status.

One of the things it skips is calling op_release. This could cause a

memory leak in the layoutget codepath if there is an error at an

inopportune time.

Have the compound processing engine always call op_release, even when

op_func sets an error in op->status. With this change, we also need

nfsd4_block_get_device_info_scsi to set the gd_device pointer to NULL

on error to avoid a double free.