libceph: fix invalid unsigned->signed conversion for timespec encoding

__kernel_time_t is a long, which cannot hold a U32_MAX on 32-bit
architectures.  Just drop this check as it has limited value.

This fixes a crash like:

[  957.905812] kernel BUG at /srv/autobuild-ceph/gitbuilder.git/build/include/linux/ceph/decode.h:164!
[  957.914849] Internal error: Oops - BUG: 0 [#1] SMP ARM
[  957.919978] Modules linked in: rbd libceph libcrc32c ipmi_devintf ipmi_si ipmi_msghandler nfsd nfs_acl auth_rpcgss nfs fscache lockd sunrpc
[  957.932547] CPU: 1    Tainted: G        W     (3.9.0-ceph-19bb6a83-highbank #1)
[  957.939881] PC is at ceph_osdc_build_request+0x8c/0x4f8 [libceph]
[  957.945967] LR is at 0xec520904
[  957.949103] pc : [<bf13e76c>]    lr : [<ec520904>]    psr: 20000153
[  957.949103] sp : ec753df8  ip : 00000001  fp : ec53e100
[  957.960571] r10: ebef25c0  r9 : ec5fa400  r8 : ecbcc000
[  957.965788] r7 : 00000000  r6 : 00000000  r5 : ffffffff  r4 : 00000020
[  957.972307] r3 : 51cc8143  r2 : ec520900  r1 : ec753e58  r0 : ec520908
[  957.978827] Flags: nzCv  IRQs on  FIQs off  Mode SVC_32  ISA ARM  Segment user
[  957.986039] Control: 10c5387d  Table: 2c59c04a  DAC: 00000015
[  957.991777] Process rbd (pid: 2138, stack limit = 0xec752238)
[  957.997514] Stack: (0xec753df8 to 0xec754000)
[  958.001864] 3de0:                                                       00000001 00000001
[  958.010032] 3e00: 00000001 bf139744 ecbcc000 ec55a0a0 00000024 00000000 ebef25c0 fffffffe
[  958.018204] 3e20: ffffffff 00000000 00000000 00000001 ec5fa400 ebef25c0 ec53e100 bf166b68
[  958.026377] 3e40: 00000000 0000220f fffffffe ffffffff ec753e58 bf13ff24 51cc8143 05b25ed2
[  958.034548] 3e60: 00000001 00000000 00000000 bf1688d4 00000001 00000000 00000000 00000000
[  958.042720] 3e80: 00000001 00000060 ec5fa400 ed53d200 ed439600 ed439300 00000001 00000060
[  958.050888] 3ea0: ec5fa400 ed53d200 00000000 bf16a320 00000000 ec53e100 00000040 ec753eb8
[  958.059059] 3ec0: ec51df00 ed53d7c0 ed53d200 ed53d7c0 00000000 ed53d7c0 ec5fa400 bf16ed70
[  958.067230] 3ee0: 00000000 00000060 00000002 ed53d200 00000000 bf16acf4 ed53d7c0 ec752000
[  958.075402] 3f00: ed980e50 e954f5d8 00000000 00000060 ed53d240 ed53d258 ec753f80 c04f44a8
[  958.083574] 3f20: edb7910c ec664700 01ade920 c02e4c44 00000060 c016b3dc ec51de40 01adfb84
[  958.091745] 3f40: 00000060 ec752000 ec753f80 ec752000 00000060 c0108444 00000007 ec51de48
[  958.099914] 3f60: ed0eb8c0 00000000 00000000 ec51de40 01adfb84 00000001 00000060 c0108858
[  958.108085] 3f80: 00000000 00000000 51cc8143 00000060 01adfb84 00000007 00000004 c000dd68
[  958.116257] 3fa0: 00000000 c000dbc0 00000060 01adfb84 00000007 01adfb84 00000060 01adfb80
[  958.124429] 3fc0: 00000060 01adfb84 00000007 00000004 beded1a8 00000000 01adf2f0 01ade920
[  958.132599] 3fe0: 00000000 beded180 b6811324 b6811334 800f0010 00000007 2e7f5821 2e7f5c21
[  958.140815] [<bf13e76c>] (ceph_osdc_build_request+0x8c/0x4f8 [libceph]) from [<bf166b68>] (rbd_osd_req_format_write+0x50/0x7c [rbd])
[  958.152739] [<bf166b68>] (rbd_osd_req_format_write+0x50/0x7c [rbd]) from [<bf1688d4>] (rbd_dev_header_watch_sync+0xe0/0x204 [rbd])
[  958.164486] [<bf1688d4>] (rbd_dev_header_watch_sync+0xe0/0x204 [rbd]) from [<bf16a320>] (rbd_dev_image_probe+0x23c/0x850 [rbd])
[  958.175967] [<bf16a320>] (rbd_dev_image_probe+0x23c/0x850 [rbd]) from [<bf16acf4>] (rbd_add+0x3c0/0x918 [rbd])
[  958.185975] [<bf16acf4>] (rbd_add+0x3c0/0x918 [rbd]) from [<c02e4c44>] (bus_attr_store+0x20/0x2c)
[  958.194850] [<c02e4c44>] (bus_attr_store+0x20/0x2c) from [<c016b3dc>] (sysfs_write_file+0x168/0x198)
[  958.203984] [<c016b3dc>] (sysfs_write_file+0x168/0x198) from [<c0108444>] (vfs_write+0x9c/0x170)
[  958.212768] [<c0108444>] (vfs_write+0x9c/0x170) from [<c0108858>] (sys_write+0x3c/0x70)
[  958.220768] [<c0108858>] (sys_write+0x3c/0x70) from [<c000dbc0>] (ret_fast_syscall+0x0/0x30)
[  958.229199] Code: e59d1058 e5913000 e3530000 ba000114 (e7f001f2)

CC: stable@vger.kernel.org  # 3.4+
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>