net/9p: Fix sparse endian warning in trans_fd.c

net/9p: Fix sparse endian warning in trans_fd.c

Address sparse endian warning:
net/9p/trans_fd.c:932:28: warning: incorrect type in assignment (different base types)
net/9p/trans_fd.c:932:28:    expected restricted __be32 [addressable] [assigned] [usertype] s_addr
net/9p/trans_fd.c:932:28:    got unsigned long

Link: http://lkml.kernel.org/r/20200618183417.5423-1-alexander.kapshuk@gmail.com
Signed-off-by: Alexander Kapshuk <alexander.kapshuk@gmail.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>