kern linker: Do not retry loading modules on EEXIST

LINKER_LOAD_FILE() calls linker_load_dependencies() which will return
EEXIST in case the module to be loaded has already been compiled into
the kernel. Since the format of the module is now recognized then there
is no need to retry loading with a different linker, otherwise the
userland will get misleading error number ENOEXEC.

PR: 274936
Reviewed by: dfr
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D42474

(cherry picked from commit ecf710f0e04e3314a492747124166ccb7cf4019e)
(cherry picked from commit e7fd435d3d4e888d9894d8c212c29ae6e2768f74)
(cherry picked from commit d18326a7cf14804086550c7463be9721e8d28b64)