imgact_elf: do not relock the text vnode if possible.

We unlock the vnode around malloc(M_WAITOK), to make it possible for
pagedaemon to flush vnode pages for us. Instead of doing it
unconditionally, first try M_NOWAIT allocation, which typically
succeed. Only on failure, unlock the vnode and retry with M_WAITOK.

Reviewed by: markj, trasz
Tested by: mjg, pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D19923