Provide inline versions of some allocation functions
 - bch2_alloc_sectors_done_inlined()
 - bch2_alloc_sectors_append_ptrs_inlined()

and use them in the core IO path.

Also, inline bch2_extent_update_i_size_sectors() and
bch2_bkey_append_ptr().

In the core write path, function call overhead matters - every function
call is a jump to a new location and a potential cache miss.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>