Just pass NULL as locked_page in case of first block in the indirect
chain.  Old calling conventions aside, a reason for having 'phys'
was that ufs_inode_getfrag() used to be able to do _two_ allocations
- indirect block and extending/reallocating a tail.  We needed
locked_page for the latter (it's a data), but we also needed to
figure out that indirect block is metadata.  So we used to pass
non-NULL locked_page in all cases *and* used NULL phys as
indication of being asked to allocate an indirect.

With tail unpacking taken into a separate function we don't need
those convolutions anymore.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>