bcachefs: DARRAY_PREALLOCATED()

Add support to darray for preallocating some number of elements.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: Switch darray to kvmalloc()

We sometimes use darrays for quite large buffers - the btree write
buffer in particular needs large buffers, since it must be sized to hold
all the write buffer keys outstanding in the journal.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

bcachefs: Factor out darray resize slowpath

Move the slowpath (actually growing the darray) to an out-of-line
function; also, add some helpers for the upcoming btree write buffer
rewrite.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>