fs/zfs/zfs: zfs_mount() - avoid pointer downcasting

fs/zfs/zfs: zfs_mount() - avoid pointer downcasting

Coverity reports that while loopis in the following functions uses
tainted data as boundary:
  zfs_mount() -> check_mos_features() -> dnode_get() -> zfs_log2()
  zfs_mount() -> grub_memmove()

The defect type is "Untrusted loop bound" caused as a result of
"tainted_data_downcast". Coverity does not like the pointer downcast
here and we need to address it.

We believe Coverity flags pointer downcast for the following two
reasons:
1. External data: The pointer downcast could indicate that the source is
  external data, which we need to further sanitize - such as verifying its
  limits. In this case, the data is read from an external source, which is
  a disk. But, zio_read(), which reads the data from the disk, sanitizes it
  using a checksum. checksum is the best facility that ZFS offers to verify
  external data, and we don't believe a better way exists. Therefore, no
  further action is possible for this.

2. Corruption due to alignment: downcasting a pointer from a strict type
  to less strict type could result in data corruption. For example, the
  following cast would corrupt because uint32_t is 4-byte aligned, and
  won't be able to point to 0x1003 which is not 4-byte aligned.
    uint8_t *ptr = 0x1003;
    uint32_t *word = ptr; (incorrect, alignment issues)

  This patch converts the "osp" pointer in zfs_mount() from a "void" type
  to "objset_phys_t" type to address this issue.

We are not sure if there are any other reasons why Coverity flags the
downcast. However, the fix for alignment issue masks/suppresses any
other issues from showing up.

Fixes: CID 314023
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>