Fix memory leak in write_pool_threads

kmemleak produces the following warning

unreferenced object 0xc9ec02a0 (size 8):
  comm "cat", pid 19048, jiffies 730243
  backtrace:
    [<c01bf970>] create_object+0x100/0x240
    [<c01bfadb>] kmemleak_alloc+0x2b/0x60
    [<c01bcd4b>] __kmalloc+0x14b/0x270
    [<c02fd027>] write_pool_threads+0x87/0x1d0
    [<c02fcc08>] nfsctl_transaction_write+0x58/0x70
    [<c02fcc6f>] nfsctl_transaction_read+0x4f/0x60
    [<c01c2574>] vfs_read+0x94/0x150
    [<c01c297d>] sys_read+0x3d/0x70
    [<c0102d6b>] sysenter_do_call+0x12/0x32
    [<ffffffff>] 0xffffffff

write_pool_threads() only frees nthreads on error paths, in the success case
we leak it.

Signed-off-by: Eric Sesterhenn <eric.sesterhenn@lsexperts.de>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>