9pfs: Fix some return statements in the synth backend

9pfs: Fix some return statements in the synth backend

The qemu_v9fs_synth_mkdir() and qemu_v9fs_synth_add_file() functions
currently return a positive errno value on failure. This causes
checkpatch.pl to spit several errors like the one below:

ERROR: return of an errno should typically be -ve (return -EAGAIN)
+        return EAGAIN;

Simply change the sign. This has no consequence since callers
assert() the returned value to be equal to 0.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <166930551818.827792.10663674346122681963.stgit@bahia>
[C.S.: - Resolve conflict with 66997c42e02c. ]
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>