btrfs: free checksum hash on in close_ctree

fs_info::csum_hash gets initialized in btrfs_init_csum_hash() which is
called by open_ctree().

But it only gets freed if open_ctree() fails, not on normal operation.

This leads to a memory leak like the following found by kmemleak:
unreferenced object 0xffff888132cb8720 (size 96):

  comm "mount", pid 450, jiffies 4294912436 (age 17.584s)
  hex dump (first 32 bytes):
    04 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<000000000c9643d4>] crypto_create_tfm+0x2d/0xd0
    [<00000000ae577f68>] crypto_alloc_tfm+0x4b/0xb0
    [<000000002b5cdf30>] open_ctree+0xb84/0x2060 [btrfs]
    [<0000000043204297>] btrfs_mount_root+0x552/0x640 [btrfs]
    [<00000000c99b10ea>] legacy_get_tree+0x22/0x40
    [<0000000071a6495f>] vfs_get_tree+0x1f/0xc0
    [<00000000f180080e>] fc_mount+0x9/0x30
    [<000000009e36cebd>] vfs_kern_mount.part.11+0x6a/0x80
    [<0000000004594c05>] btrfs_mount+0x174/0x910 [btrfs]
    [<00000000c99b10ea>] legacy_get_tree+0x22/0x40
    [<0000000071a6495f>] vfs_get_tree+0x1f/0xc0
    [<00000000b86e92c5>] do_mount+0x6b0/0x940
    [<0000000097464494>] ksys_mount+0x7b/0xd0
    [<0000000057213c80>] __x64_sys_mount+0x1c/0x20
    [<00000000cb689b5e>] do_syscall_64+0x43/0x130
    [<000000002194e289>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Free fs_info::csum_hash in close_ctree() to avoid the memory leak.

Fixes: 6d97c6e3 ("btrfs: add boilerplate code for directly including the crypto framework")
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>