ceph: fix memory leak in __ceph_setxattr()

The ceph_inode_xattr needs to be released when removing an xattr.  Easily
reproducible running the 'generic/020' test from xfstests or simply by
doing:

  attr -s attr0 -V 0 /mnt/test && attr -r attr0 /mnt/test

While there, also fix the error path.

Here's the kmemleak splat:

unreferenced object 0xffff88001f86fbc0 (size 64):
  comm "attr", pid 244, jiffies 4294904246 (age 98.464s)
  hex dump (first 32 bytes):
    40 fa 86 1f 00 88 ff ff 80 32 38 1f 00 88 ff ff  @........28.....
    00 01 00 00 00 00 ad de 00 02 00 00 00 00 ad de  ................
  backtrace:
    [<ffffffff81560199>] kmemleak_alloc+0x49/0xa0
    [<ffffffff810f3e5b>] kmem_cache_alloc+0x9b/0xf0
    [<ffffffff812b157e>] __ceph_setxattr+0x17e/0x820
    [<ffffffff812b1c57>] ceph_set_xattr_handler+0x37/0x40
    [<ffffffff8111fb4b>] __vfs_removexattr+0x4b/0x60
    [<ffffffff8111fd37>] vfs_removexattr+0x77/0xd0
    [<ffffffff8111fdd1>] removexattr+0x41/0x60
    [<ffffffff8111fe65>] path_removexattr+0x75/0xa0
    [<ffffffff81120aeb>] SyS_lremovexattr+0xb/0x10
    [<ffffffff81564b20>] entry_SYSCALL_64_fastpath+0x13/0x94
    [<ffffffffffffffff>] 0xffffffffffffffff

Cc: stable@vger.kernel.org
Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>