nvmet: remove redundant assignment of error code in nvmet_ns_enable()

Remove the unnecessary ret = -EMFILE; assignment since it is immediately
overwritten by the result of nvmet_bdev_ns_enable() The initial value
(-EMFILE) is redundant because it has no effect on the code logic or
outcome.

Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>