devfs: tidy up after 272596

This moves a var to an if statement, no functional changes.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272600 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
MFV r272591:

Use loaned ARC buffer for zfs receive to avoid copy.

Illumos issue:
    5162 zfs recv should use loaned arc buffer to avoid copy

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272601 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f